Omarthis, Ghostfire Initiate is a mythic Legendary Creature — Spirit Snake card from Commander Masters (CMM), collector number 748. Current market price: $3.75 (foil $4.48), TCGplayer market prices via Scryfall.
Omarthis enters with X +1/+1 counters on it. Whenever you put one or more +1/+1 counters on another colorless creature, you may put a +1/+1 counter on Omarthis. When Omarthis dies, manifest a number of cards from the top of your library equal to the number of counters on it.