Cheapest priced copy (Commander 2020): $7.99 at Card Kingdom, near mint, their price list as of .
Altaïr, Brotherhood Mentor 
Kelsien, the Plague
Legendary Creature — Human Assassin · 2/2 · Mythic · Secret Lair Drop · #1558
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.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
2 printings · $0.99 to $13.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander 2020 | 11 | Mythic | $7.99 | $0.99 |
| Secret Lair Drop (Altaïr, Brotherhood Mentor) | 1558 | Mythic | $10.99 | $13.99 |
3 rulings from Wizards of the Coast
- 17 April 2020If the target of Kelsien's last ability dies before the ability resolves, you won't get an experience counter.
- 17 April 2020Each 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.
- 17 April 2020If Kelsien leaves the battlefield, you keep any experience counters you've received.