Kelsien, the Plague is a mythic Legendary Creature — Human Assassin card from Secret Lair Drop (SLD), collector number 1558. Current market price: $14.54, 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.