Gray Merchant of Asphodel

Mana cost: {3}{B}{B}
Type: Creature — Zombie
Power / toughness: 2 / 4
Set: The Zeta Set #285 (common)

When this creature enters, each opponent loses X life, where X is your devotion to black. You gain life equal to the life lost this way. (Each {B} in the mana costs of permanents you control counts toward your devotion to black.)

Gray Merchant of Asphodel has 14 printings, first in Theros (2013). EDHREC ranks it #241 among Commander cards. It appears most in K'rrik, Son of Yawgmoth decks (18,191 on EDHREC). 4 public decks on TCG run it. It has 6 official rulings.

Legality

commander: legal
legacy: legal
vintage: legal
pauper: legal
modern: legal
pioneer: legal
standard: not legal
brawl: legal

Public decks that run Gray Merchant of Asphodel

Endless Punishment - Duskmourn: House of Horror Commander, led by Valgavoth, Harrower of Souls
Grave Danger - Starter Commander Decks, led by Gisa and Geralf
Sworn to Darkness - Commander 2014, led by Ob Nixilis of the Black Oath
Wretched Ranks - Foundations Commander, led by Ghoulcaller Gisa

Commanders that run Gray Merchant of Asphodel most

Deck counts from EDHREC.

K'rrik, Son of Yawgmoth: 18,191 decks
Wilhelt, the Rotcleaver: 16,636 decks
Sephiroth, Fabled SOLDIER // Sephiroth, One-Winged Angel: 15,720 decks
Valgavoth, Harrower of Souls: 14,136 decks
Meren of Clan Nel Toth: 13,326 decks
The Scarab God: 9,584 decks
Sheoldred, the Apocalypse: 7,379 decks
Varina, Lich Queen: 6,666 decks

Often played with Gray Merchant of Asphodel

Cards that show up in the same decks on EDHREC.

Rulings

2020-01-24: If you put an Aura on an opponent's permanent, you still control the Aura, and mana symbols in its mana cost count towards your devotion.
2020-01-24: Colorless and generic mana symbols ({C}, {0}, {1}, {2}, {X}, and so on) in mana costs of permanents you control don't count toward your devotion to any color.
2020-01-24: If an activated ability or triggered ability has an effect that depends on your devotion to a color, you count the number of mana symbols of that color among the mana costs of permanents you control as the ability resolves. The permanent with that ability will be counted if it's still on the battlefield at that time.
2020-01-24: Hybrid mana symbols, monocolored hybrid mana symbols, and Phyrexian mana symbols do count toward your devotion to their color(s).
2020-01-24: Mana symbols in the text boxes of permanents you control don't count toward your devotion to any color.
2021-03-19: The amount of life you gain is the total amount of life lost, not simply the value of X.