Golden Guardian

2018-01-19 For more information on double-faced cards, see the Ixalan mechanics article (http://magic.wizards.com/en/articles/archive/feature/ixalan-mechanics).
2018-01-19 If Golden Guardian leaves the battlefield before its activated ability has resolved, it won't be returned to the battlefield when the ability resolves.
2018-01-19 If the target of Golden Guardian's activated ability isn't a legal target as that ability resolves, or if Golden Guardian has left the battlefield, neither creature will deal or be dealt damage.
2018-01-19 Once Golden Guardian's activated ability has resolved, it will return to the battlefield transformed if it dies for any reason in that turn.

Card is in preconstructed decks:

mtg.wtf