During the Whitsun holidays in the year 1070, Heinrich IV stayed in Fritzlar. Whilst there, a minor nobleman of ill repute appeared on the scene by the name of Egeno. He raised a [serious] charge against Otto von Nordheim. He claimed that he had been hired by Otto to kill the king. As evidence, he presented the sword which had allegedly been given to him to carry out this very act to all those present at the time.
 The actual word used by KHW for the mastermind's tactics is 'drahtzieher', which means literally 'cable puller'. In this context it seems likely that KHW is referring to the man who pulled the strings.
 An 'enfeoff' is a medieval term meaning that lands were given in return for pledged service. It's a derivative of 'fief'.
 The Harz Mountains are a range in northern Germany which cover the south-eastern edge of Lower Saxony and the western edges of Saxony-Anhalt and Thuringia. In medieval times, and since the time of the Carolingians, it was a royal hunting preserve rather like the New Forest in England and very much a 'hands off' area (this was undoubtedly due to much of the Harz being forest, as mentioned in contemporary documents).
This was a very serious charge (and one that could not be ignored). Despite the accused Otto maintaining his innocence he was nevertheless challenged to a decisive duel (combat à l'outrance),  with Egeno [the outcome of which would be considered to be] the judgement of God.  The duel would take place in the presence of the king. The duke of Bavaria was held in high esteem by the princes,  and this demand that he should restore his honour and clear himself of the charge through a duel with someone who was regarded as being little better than a common highwayman caused feelings [of indignation] to run high.
When Otto firstly made impossible demands (Gebhard 25I), and then refused to appear altogether, Heinrich imposed his decree, stripping Otto of the duchy of Bavaria and adding Otto's possessions to his own. Since Frankish times this had been a legal consequence of such a refusal. Any person so sentenced lost his right to protection, became an outlaw - in German 'the friend of wolves' - and like any wolf or bird could be killed with impunity.
Otto resorted to arms and after allying himself with Magnus Billung, son of the duke of Saxony, went forth into battle [but alas to no avail], both being beaten by the beginning of 1071, forced to submit to the mercy of the king, and taken into custody [imprisoned].
The answer is simple. Count Giso of Hollende and Count Adalbert of Schauenburg were the instigators, movers, and shakers of a very nasty plot. Either acting in support of Heinrich IV, or perhaps even acting on his orders, they put a plan together, formulated the accusation [against Otto] using their already-paid-for tool, Egeno, and then went public. The aim was the complete disempowerment of the duke of Bavaria and the expropriation of his entire possessions, which would have made direct access to Saxony and Thuringia an easy matter for Heinrich IV.
As a result of the intercession of Archbishop Adalbert of Bremen, Otto was pardoned a year later and, while he had to give up some of his fiefs, he nevertheless held on to his family estate which had been acquired before the feudal system, with only Magnus Billung remaining in prison. The conflict, however, was far from over and after discussions with the Saxons it had become a case of hanging fire - ie. forming a truce.
When in 1073 King Heinrich invited others to join him in a campaign against Poland, the Saxon princes (fürsten) demanded that before any participation on their part, he dealt with their complaints first. As a result of the king rejecting this demand, the Saxons formulated their own conspiracy, their leader being none other than Otto von Nordheim.
When the king sent an embassy in an effort to stall proceedings, the immediate response was to get an army together and attack Heinrich in Harzburg [Grosse Harzburg is a former imperial castle on the western edge of the Harz Mountains], where he was staying at the time. Heinrich was obliged to flee to Hessen (directly south where he still had possessions), as he was not well placed to meet the strong Saxons in battle. The insurgency spread all over Saxony and Thuringia with a number of important castles falling into enemy hands (Gebhard 251).
 A duel or combat à l'outrance (literally to the utmost) ended only in the death of one of the parties. It was the common way of settling such disputes in the Middle Ages until, in England at least, the legal system had been properly encoded. As it is, duelling over disputes still carried on despite becoming illegal and still happens today.
 In the matter of the judgement of God, it was considered at the time that, whatever the outcome, God had delivered a judgement which could not be disputed - at least not openly. Such duels are mentioned in historical fiction, notably by Sir Walter Scott (Ivanhoe and Quentin Durward) and Ellis Peters (Brother Cadfael - One Corpse Too Many). In the case of women, they could not fight themselves but were allowed to appoint what was known as a champion to fight on their behalf, which is the case in the novel Ivanhoe. In medieval Spain, fights between rival royals was often handled through royal champions - El Cid was one of these.
 In German there are two terms for 'prince': 'Der Prinz' and 'Der Furst', the latter being the ruler of what is known as a principality such as in the cases of Liechtenstein and Monaco. In Germany, Prince Charles of Britain is often referred to as 'Der Furst von Wales'.
Count Giso and Adalbert, along with their four sons whether because of (high-ranking) enemies or more personal hostilities, were killed in the castle of Hollende that pertained to Giso.
Otto von Nordheim had, through this bloody deed, finally rid himself of the author(s) of his misfortune. Even Egeno, Otto's accuser, was not spared an inglorious end. According to Lampert von Herfseld, he was taken after another highway robbery, blinded, and left for the rest of his days as a beggar, going from from house to house.
Thanks to this literary [documented] account of Giso von Hollende's death there can be no doubt that by this time Burg Hollende had become the family seat and can therefore certainly be considered to be the actual home of the Giso dynasty (Diefenbach 115).
 Lampert of Hersfeld under his English name Lambert has an entry on German Wikipedia. According to this, he spent his early years in a monastery in Aschaffenburg in Bavaria, but at the same time close to the border with Hessen, and ended his life as abbot of Hasungen near Kassel. He is also sometimes referred to as Lampert von Aschaffenburg. The German Wiki is also somewhat more informative in mentioning the archbishop of Mainz who was also mentioned in Gisones Part 1. According to its local history, Aschaffenburg was part of the archbishopric's domain for eight hundred years. Also according to the German Wiki, Lampert was a persuasive opponent of Heinrich IV.
 The Latin word, 'occisi', from the verb 'occidere', meaning 'to kill'. In Italian the verb is 'uccidere', which is used multiple times in the Italian tv series, Inspector Montalbano, and conveys the meaning of 'murder'. KHW translates it as 'geschlagen', which can mean 'murdered' or 'slaughtered', or 'massacred' rather than 'getötet', meaning 'killed'. The normal term for battle is 'Der Kampf'; but very often when it's rather gruesome or bloody, particularly in a military context, the word 'Der Schlacht' is used, meaning 'slaughter / massacre'. Given who was behind the attack, KHW seems to have this just right.