Kelsien, the Plague is a mythic Legendary Creature — Human Assassin card from Commander 2020 (C20), collector number 11. Current market price: $2.47 (foil $0.94), TCGplayer market prices via Scryfall.
Vigilance, haste Kelsien gets +1/+1 for each experience counter you have. {T}: Kelsien deals 1 damage to target creature you don't control. When that creature dies this turn, you get an experience counter.