Altair, Brotherhood Mentor - Kelsien, the Plague (SLD-1558) - Secret Lair Drop / Altaïr, Brotherhood Mentor (Borderless)

Vigilance, haste
Kelsien gets +1/+1 for each experience counter you have.
: Kelsien deals 1 damage to target creature you don't control. When that creature dies this turn, you get an experience counter.