Altar of Dementia

• WWizards of the Coast2019-06-14
  Official Ruling
  If the target player's library contains fewer cards than the sacrificed creature's power, all of those cards are put into that player's graveyard. That player won't lose the game until they try to draw from the empty library.
• WWizards of the Coast2019-06-14
  Official Ruling
  The number of cards the target player puts into their graveyard is equal to the power of the sacrificed creature as it last existed on the battlefield.

Rulings via Scryfall