What is the "Timeshifted" rarity?
Timeshifted were rather special. Every booster pack from the Time Spiral block had one Timeshifted card, and each Timeshifted card was equally likely to be the included card.

Quoting Wikipedia,


  The Timeshifted cards are distributed one per booster pack and three per tournament pack (taking the place of common cards), and also appear in the preconstructed decks. According to DCI-distributed tournament primers, Timeshifted cards are tournament legal where Time Spiral or the set of original printing are.


This also means that since the Modern format includes Time Spiral, it includes Undertaker.