Thraximundar is a mythic Legendary Creature — Zombie Assassin card from Secret Lair Drop (SLD), collector number 142. Current market price: $11.22, TCGplayer market prices via Scryfall.
Haste Whenever Thraximundar attacks, defending player sacrifices a creature of their choice. Whenever a player sacrifices a creature, you may put a +1/+1 counter on Thraximundar.