Hearthstone's development team has decided that the Undertaker card is too harmful to the game's balance. They're going to significantly weaken the card in a future patch.
Undertaker is a 1 attack, 2 health minion with huge upside. Whenever you summon a minion with Deathrattle, the Undertaker gains 1 attack and 1 health. If you play an Undertaker early and have a lot of Deathrattle minions in your deck, you could easily turn it into a juggernaut. This is a very viable strategy, considering how many Deathrattle cards were added to the game with the Curse of Naxxramas expansion. Here's a video of an Undertaker hunter deck in action:
When the patch goes live, Undertaker will only gain 1 attack and no health from playing Deathrattle minions. This will make it much easier for opponents to remove them from the battlefield without expending too much of their mana supply to do it.
"Undertaker was frustrating to play against," said Blizzard. "It often gained both Attack and Health stats significantly above those of other inexpensive minions very early in the game. With this change, we expect Undertaker will still be better than other 1-Mana minions when played in a deck with a Deathrattle theme, but more likely to die in combat against other minions."
If you crafted the golden version of Undertaker, you'll be able to disenchant it for the full cost (400 dust) once the nerf patch is live. This full refund will only be offered for a short period of time, though.
Blizzard said in December that they weren't concerned about Undertaker. Senior game designer Ben Brode noted in an interview that it's only a potent card if you draw him very early in the match.
"The Undertaker is one of those cards that feels really powerful when he's hurting you, but if you draw him late in the game he's pretty terrible," Brode said at the time. "We were paying attention to how people were using him, and looked at how we were playing against him, but we didn't feel he's nerf-worthy. He is very powerful, but it wasn't past the line for us. I think it's worth noting that Kolento recently won a pretty large tournament and didn't use Undertaker in any of his decks."
Brode also noted that they prefer not to nerf cards. They've only weakened a handful of cards since the game's official launch last year. Their usual approach is to add more cards and hope players develop strategies to neutralize "overpowered" decks.
It seems that the development team was hoping that the new cards introduced in the Goblins vs. Gnomes expansion would make Undertaker less dominant. That didn't happen so they decided to finally reduce the card's effectiveness.