"There is no greater glory than fighting to find the truth."

Kelsien, the Plague

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.

Secret Lair Drop

"Aim for an eye. Better they never see you."

Commander 2020

Vigilance, haste
Kelsien, the Plague 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.

If the target of Kelsien's last ability dies before the ability resolves, you won't get an experience counter.

Each time Kelsien's last ability resolves, it creates a separate delayed triggered ability. If you untap Kelsien and activate its last ability multiple times in one turn, they'll each trigger when the appropriate creature or creatures die. 

If Kelsien leaves the battlefield, you keep any experience counters you've received.