Source: Blizzard
I haven't been playing Hearthstone for a while because games just feel so similar to me with the relatively small card pool. Games just seem to go a certain "fixed" way where you or your opponent draw a series of cards to play and win. (Edit: The guaranteed mana system kind of help out too) Anyway, Undertaker no longer gains health from the the next patch and I know a lot of players dislike the card so it is probably quite a welcomed change. It is one of the cards that when drawn early with you having the death-rattle cards to support it along with your opponent having no answer soon enough, it usually results in a win.
