Velka, Sister to Vosk

Skulk, Trample
Instability (At the beginning of your upkeep, you may pay {x}. If you do, this creature has base power and toughness X/X, and it can't have -1/-1 counters placed on it.)
Whenever Velka deals combat damage to a player that player puts the top X cards of his or her library into his or her graveyard. Where X is the damage dealt multiplied by Velka's power.