Grenzo, Dungeon Warden enters the battlefield with X +1/+1 counters on it.
: Put the bottom card of your library into your graveyard. If it's a creature card with power less than or equal to Grenzo's power, put it onto the battlefield.
3/16/2018 Compare Grenzo’s power when the ability resolves with the power of the creature card in your graveyard to determine if you put it onto the battlefield. If Grenzo isn’t on the battlefield at this time, use its last known power from when it was on the battlefield.
3/16/2018 If the card you put into your graveyard isn’t a creature card or it’s a creature card with power greater than Grenzo’s power, it stays in your graveyard.