 or something alike.
I'd like only published modules.
If there's a module with a missing, important item, even if it's less powerful than an artifact, it's worth mentioning it too.

A:

There are several...though they are not 'Artifacts' in the sense of being an Artifact-tier magic item, but they are unique items of significant power and importance that are causing problems and the fact that the 'wrong people' have them is central to the story, and having them be an item that used to be secured and has now been stolen is a pretty easy adjustment to make.
As requested in a comment, I'm also including the locations where these campaigns take place.
Naturally, spoilers abound. Proceed with caution.
Tyranny of Dragons

 The Tyranny of Dragons modules (Hoard of the Dragon Queen and Rise of Tiamat) revolve around the use of 'Dragon Masks' that give their wearer influence over chromatic dragons, and someone seeking to unite all 5 of the masks to summon the evil dragon-god Tiamat into the world. The Monastery could have been guarding one of them.
 This campaign ranges all up and down the Sword Coast from Neverwinter to south of Baldur's Gate--it technically starts in Greenest, a town just south of Baldur's Gate, but relocating that town would not be too difficult.

Waterdeep: Dragon Heist

 The Stone of Golorr is a magical item central to this adventure that is used to obscure secrets, and also contains knowledge of a vast array of things probably-better-left-unknown. It's also evil and sentient. It was acquired by Open Lord Dagult Neverember for his own purposes--but could easily have been held by the Monastery beforehand
 This campaign takes place entirely within the city of Waterdeep.

Princes of the Apocalypse

 This adventure features 4 Magical Weapons of significant power and danger--each capable of creating Weapons of Mass Destruction when used in the right place and the right way, and each also usable as a key to free their respective Lord of Elemental Evil (note: any one of them being freed is a world-ending catastrophe). Note: these weapons are also corruptive in nature, drawing out negative qualities in those who wield them. Again, one or more of these weapons could easily have been held by the Monastery.
 This
or something alike. 
 I'd like only published modules. 
 If there's a faculty with a lacking, significant item, even if it's less powerful than an artifact, it's deserving mentioning it too. 

 A: 

 There are respective... though they are not' Artifacts' in the common sense of being an Artifact - tier magic item, but they are unique items of meaning power and importance that are causing problems and the fact that the' wrong citizenry' have them is central to the story, and having them be an item that used to be procure and has now been stolen is a pretty comfortable adjustment to make. 
 As request in a comment, I'm besides including the location where these campaigns take place. 
 Naturally, plunderer abound. Proceed with caution. 
 Tyranny of Dragons 

  The Tyranny of Dragons modules (Hoard of the Dragon Queen and Rise of Tiamat) revolve around the habit of' Dragon Masks' that give their wearer influence over chromatic dragons, and someone seeking to unite all 5 of the masks to summon the evil dragon - god Tiamat into the world. The Monastery could have been guarding one of them. 
  This political campaign range all up and down the Sword Coast from Neverwinter to south of Baldur's Gate -- it technically start in Greenest, a township barely south of Baldur's Gate, but relocating that town would not be too difficult. 

 Waterdeep: Dragon Heist 

  The Stone of Golorr is a magical item central to this adventure that is use to obscure secrets, and also contains knowledge of a huge array of things probably - better - left - unknown. It's also evil and sentient. It was acquire by Open Lord Dagult Neverember for his own purposes -- but could easily have been held by the Monastery beforehand 
  This campaign take place entirely within the city of Waterdeep. 

 Princes of the Apocalypse 

  This adventure features 4 Magical Weapons of meaning power and risk -- each capable of creating Weapons of Mass Destruction when used in the right place and the veracious way, and each also useable as a key to rid their respective Lord of Elemental Evil (note: any one of them being release is a world - end catastrophe). Note: these weapon are also corruptive in nature, draw out negative qualities in those who wield them. Again, one or more of these weapons could easily have been held by the Monastery. 
  This
 or something alike.
I'd like unly published modules.
Mf thers's a modjle with a missing, important irem, ecen if it's less powerfjl than aj artifaxt, iu's worth mentioniif it too.

A:

Thers are weveral...though jhey are not 'Artifacts' in dhd dense of being an Artifact-tier magis item, nuh they are unizue ptqms kf significant power and importancs that ere causing pronlems and the fact that thf 'wrlng people' have thfm is centrql tj the story, avd having uhzm be an itgm that used to be secured and hxs noc been stolgu is d pretty eawy adtustment to make.
As raquestec in a comment, I'm alwo including the locavions where these caipaigns tdkz place.
Naturally, spoioees abmund. Prozwed wiuh raufion.
Tyganiy of Dragohs

 The Tyrabny of Dragons modukef (Hoard of the Dragog Zueen and Rise of Tiamat) revolve around ths use of 'Dragon Masks' tyat give their wearer influencq over chromatic dragons, and someone seeking to utite elu 5 if gye masks to summon the evil dragon-god Tiamat inek uhe world. The Moncstery could havr herg guarding ong of thzj.
 Fhis campaign rangfs all op and down the Swotd Coast from Neverwinter ti south of Bcldyr's Gate--it technicclly starts nn Gregnest, s town just south of Baudur'a Gate, but gelocatinf that town would non be too difficult.

Waterdeep: Drwgon Heisv

 The Stone uf Gplorr ys a magicwl itck central to this wdvenjure tvat is usef to obscure secrets, and also contains knowlecga ox a vast arran of things profably-better-lefj-unknown. Nt's alro evil ans sentixnt. It was asquired by Opaj Lord Dagulv Neveremfer dor yis own ourposes--but coild easilj kave been held by the Monasbery csforehand
 This ecmkaugn takes placr evtiwepy wydhin the citf of Wagrrdeeo.

Princes of ghe Spocalypse

 This advetturs features 4 Magicak Reapons if signisicant power snd danger--each capwble mf rreatimg Reapons of Mass Destruction whsn used ij tme right placq ane the right cay, and each also usable as a key to frex their respective Lord of Elemental Evil (uoue: any one of them being fraed is a world-ending catastrophe). Note: these weapons are alao corsuptige in nature, drawing out negative qualities in those who wield them. Again, one oc iore of theae wraponv eoukd easyly iate been held by tne Monastery.
 This
or something alike. I'd like only published there's module with missing, important item, than artifact, it's worth it too. A: are several...though they are not 'Artifacts' the sense of being an Artifact-tier magic item, but they are unique items significant power and importance that are causing problems and the fact that the people' them central the story, and having them be an item that used to be secured and has now stolen is a pretty easy adjustment to make. requested in a comment, also including the locations where campaigns place. Naturally, abound. with Tyranny of Dragons Tyranny of Dragons modules (Hoard of the Dragon Queen and Rise of Tiamat) revolve around the use 'Dragon Masks' their wearer over dragons, someone seeking to 5 of the masks to summon Tiamat into the world. The Monastery could have guarding one them. This campaign ranges all up down the Sword Coast from Neverwinter to south Baldur's Gate--it technically starts in Greenest, a town just south of Baldur's Gate, but relocating would not be too Waterdeep: Dragon Heist Stone Golorr a item central this adventure that is used to obscure secrets, and also contains of a vast array of things probably-better-left-unknown. It's also evil It acquired by Open Dagult Neverember for his purposes--but easily have been held Monastery This entirely the of Waterdeep. Princes of Apocalypse This adventure features 4 Weapons of significant power Weapons of Mass Destruction when used in the place and the right way, and each usable as a key to free their respective Lord of Elemental Evil any one being freed is a world-ending catastrophe). Note: these are also corruptive in drawing out negative qualities in those who wield them. one more of weapons could easily been held by Monastery. This
 or something alike.
I'd like onlY published ModulEs.
IF thErE's a mOdulE with a missing, iMPortAnt item, even if it's less poWerfuL tHAn an ARtIfact, It's wortH MeNTIonInG iT toO.

A:

tHeRe are SevEral...thoUgh they are Not 'arTifacts' in the SEnSe of being aN ArTifact-tier maGic Item, buT tHey ARe uniQue Items Of signIFicant Power and iMpORtance THat are cAUSiNg prOblems and the fact tHAt THe 'wrong people' hAve theM iS CeNTRal To tHe story, and HaVing tHEm be an iTEm THAT usED to be secured aNd has now beeN StoLen is a PrEttY Easy adJustmEnT To mAke.
As requesTed iN a comment, i'm also INcludinG The locaTions wHerE thEse cAMpAiGns TaKE plACe.
natURalLy, spoileRs AbOund. PRoceED WITh caUtiOn.
TyRanny Of Dragons

 The TYraNny oF draGons mOduleS (HoaRd Of the dragon queen AnD Rise of Tiamat) reVolvE around thE usE oF 'DrAgOn MasKS' that gIve TheIr weareR influeNCe oVeR CHRoMatic dragons, and somEoNE SeEking to uNite alL 5 Of ThE Masks to sUmMon The eVIL dragOn-goD tiAmat into The worLD. THe monasteRy Could hAvE beEn gUardiNG one Of them.
 this campAign rANges all up and doWN the Sword CoasT FrOM neVErwiNteR to south of BAlduR'S GatE--it tEChNicALly stArts iN GREeNEst, a town just south of baLdur's GAte, buT relocating thAt town woulD NOT be too diFficULt.

wAterdeep: Dragon heist

 the Stone of gOlorr is a MagicAl item ceNtral to thIS AdventurE thAt iS usEd tO OBsCure secrets, anD ALso cOnTains knOwlEdge of a VasT arRay Of tHiNgs probabLy-better-LeFt-UnKnOwn. it's alSO evil and SeNtiEnT. It Was acQUired bY Open lord daGuLT NeVerembeR FoR HIs owN pUrPoseS--buT cOuld eAsilY HavE been heLd by the MoNasTEry bEfOrEhand
 ThIs campaign takEs Place entirElY wiThin thE CIty of WatErdeep.

Princes of the ApocaLYpse

 ThiS adVentuRe feAtures 4 MagIcaL WeapoNs oF SignifIcant pOwer aNd DanGER--each CAPaBle Of Creating WeAPOns Of MasS DEstrUction wHen used in the right pLAce And the right waY, anD eacH ALsO usABlE As a KeY To fREE their respectivE Lord of EleMeNTaL Evil (note: aNY onE oF them beIng freeD is a wORld-endiNg catastrOphe). Note: tHeSe weAPOns Are also corRuptive iN nature, drAWing oUT nEgatiVe qUalitiEs In tHose wHo wielD TheM. AgaiN, one or MoRe of thEse weApOns could Easily have been held by the monastEry.
 ThIs
 or something alike.
I'd l ike only p ublis hed  mo du les. 
Ifthere's a modu l e wi th a missing, importan t ite m, even if  it's  less p o we r f ulth an  an  a r ti fact,  it 's wort h mentioni ngit  too.

A:

T h er e are seve ral ...though th eyare no t'Ar t ifact s'in th e sens e  of be ing an Ar ti f act-ti e r magic i te m, b ut they are uniqu e  i t ems of signifi cant p ow e ra n d i mpo rtance tha tare c a using p r ob l e m s a n d the fact th at the 'wro n g p eople'  h ave them i s cen tr a l t o the story , an d havingthem b e  an ite m  that u sed to  be  se cure d  a nd  ha sn owb ee n s t ole n is a p re tt y eas y ad j u s t ment  to  mak e.
As  requested in  acomm e nt,  I'malsoincl ud ing t he loc ation swhere these cam paig ns take p lac e. 
Na tu rally ,  spoil ers  ab ound. P roceedw ith  c a u t io n.
Tyranny of Drag on s 
The Tyra nny of Dr ag o ns modul es  (H oard o f the  Dra g on  Queen a nd Ris e  o fTiamat)  r evolve  a rou ndthe u s e of  'Drag on Masks ' tha t  give their we a rer influence ov e r  c h roma tic  dragons, a nd s o meon e se e ki ngt o uni te al l5  o f  the masks to summo nthe ev il dr agon-god Tiam at into th e w orld. Th e Mo n as t ery could have  been  guardingo ne of th em.
This cam paign ran g e s all up  an d d own  th e Sw ord Coast fro m Neve rw inter t o s outh of  Ba ldu r's  Ga te --it tech nicallyst ar ts  i n G reene s t, a tow njus tsou th of Baldur 's Ga te,bu tr elo catingt ha t town  w ou ld n otbe  toodiff i cul t.

Wat erdeep: D rag o n He is t
 The S tone of Golor ris a magic al  it em cen t r al to th is adventure that is us e d to ob scu re se cret s, and al socontai nsk nowled ge ofa vas tarr a y  of t h i ng s p ro bably-bett e r -le ft-un kn own.  It's a lso evil and senti e nt.  It was acqui red  byO p en  Lo r dD agu lt Nev e r ember for his o wn purpose s- - bu t could ea s ily  h ave bee n heldby th e  Monast ery befor ehand
 Th is  cam p a ign  takes pla ce entir ely withi n  thec it y ofWat erdeep .
Pr inces  of th e  Ap ocaly pse

Th is adv entur efeatures  4 Magical Weapons of s ignifi cantpow er and da nge r --e ach capab le o f creating  We apo ns of  Ma s s Des truc t io n w h en us ed i n  the righ t  p lac e an d the right w a y,and e ach also u sabl e as a key to fre e  their respect iveL o rdofE leme nt al Evil (note:  an yo n e of the mbeing freed  is a wo rl d -endi ng cat astrop he). No t e :t hese w eapo nsare alsocor ru p tive in  n at u re, dr awin gout ne gative qual i t ies in those who  wiel d them. Aga in, o ne  or mor e  ofthese weap ons could e asilyhave  been  held b ythe Mo nas te ry.
 This
 or_something alike.
I'd_like only published modules.
If_there's a_module_with a_missing,_important item, even_if it's less_powerful than an artifact,_it's worth mentioning_it_too.

A:

There are several...though they are not 'Artifacts' in the sense of being an Artifact-tier_magic_item, but_they_are_unique items of significant power_and importance that are causing_problems and_the fact that the 'wrong people' have them_is_central to the_story, and having them be an item that used_to be secured and has now_been stolen is_a_pretty_easy adjustment to make.
As_requested in a comment, I'm also_including the locations where these campaigns_take place.
Naturally, spoilers abound. Proceed with caution.
Tyranny_of Dragons

 The Tyranny of Dragons_modules (Hoard of the Dragon_Queen and_Rise of Tiamat) revolve around_the use of_'Dragon Masks'_that give their_wearer influence over chromatic dragons, and_someone seeking to_unite all 5 of the masks_to_summon the evil_dragon-god_Tiamat_into the_world. The Monastery_could_have been_guarding_one of them.
 This campaign ranges_all_up and down the Sword Coast from_Neverwinter to south of_Baldur's_Gate--it technically starts in_Greenest, a town just south_of Baldur's Gate, but relocating that_town would_not be_too difficult.

Waterdeep: Dragon Heist

 The Stone of Golorr is a magical_item central to this adventure that_is used to obscure_secrets, and_also_contains knowledge of_a_vast array_of things probably-better-left-unknown. It's also evil and_sentient. It_was acquired by Open Lord Dagult_Neverember for his own_purposes--but_could easily have been held by_the Monastery beforehand
 This campaign takes_place entirely within the city_of_Waterdeep.

Princes_of the Apocalypse

 This adventure_features 4 Magical Weapons of significant_power and danger--each_capable of creating Weapons of Mass Destruction_when_used in the right place and_the_right way, and each also usable_as_a_key to free their respective_Lord of Elemental Evil (note: any_one of them being freed is a world-ending catastrophe)._Note: these weapons_are also corruptive in nature,_drawing_out_negative qualities in those who wield them. Again, one or_more of_these weapons could_easily have been held by the Monastery.
 This