Erinis, Gloom Stalker — ruling 1

Choose a Background is a variant of the partner ability. You may have two commanders if one of them is a legendary creature with the choose a background ability and the other is a legendary Background enchantment. Backgrounds and cards with choose a Background do not interact with cards which have any other partner ability.

Erinis, Gloom Stalker — ruling 2

If your Commander deck has two commanders, you can include only cards whose own color identities are also found in your commanders' combined color identities.

Erinis, Gloom Stalker — ruling 3

Both commanders start in the command zone, and the remaining 98 cards (or 58 cards in a Commander Draft game) of your deck are shuffled to become your library.

Erinis, Gloom Stalker — ruling 4

If your commander loses the choose a Background ability or stops being a Background during the game, as appropriate, it is still your commander.

Erinis, Gloom Stalker — ruling 5

Once the game begins, your two commanders are tracked separately. If you cast one, you won't have to pay an additional {2} the first time you cast the other. A player loses the game after having been dealt 21 combat damage from any one of them, not from both of them combined (although your Background won't usually be a creature anyway).

Erinis, Gloom Stalker — ruling 6

If something refers to your commander while you have two commanders, it refers to one of them of your choice. If you are instructed to perform an action on your commander (e.g. put it from the command zone into your hand due to Command Beacon), you choose one of your commanders at the time the effect happens.

Erinis, Gloom Stalker — ruling 7

An effect that checks whether you control your commander is satisfied if you control one or both of your two commanders.

Erinis, Gloom Stalker — ruling 8

You can choose two commanders that are the same color or colors.

Erinis, Gloom Stalker : règles, fonctionnement et interactions (MTG)

Erinis, Gloom Stalker is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Deathtouch Whenever Erinis attacks, return target land card from your graveyard to the battlefield. Choose a Background (You can have a Background as a second commander.)

How it works : Erinis, Gloom Stalker

Original English text — Wizards of the Coast / Scryfall

#1June 10, 2022· Wizards of the Coast
Choose a Background is a variant of the partner ability. You may have two commanders if one of them is a legendary creature with the choose a background ability and the other is a legendary Background enchantment. Backgrounds and cards with choose a Background do not interact with cards which have any other partner ability.
No other card is named in this ruling.
#2June 10, 2022· Wizards of the Coast
If your Commander deck has two commanders, you can include only cards whose own color identities are also found in your commanders' combined color identities.
No other card is named in this ruling.
#3June 10, 2022· Wizards of the Coast
Both commanders start in the command zone, and the remaining 98 cards (or 58 cards in a Commander Draft game) of your deck are shuffled to become your library.
No other card is named in this ruling.
#4June 10, 2022· Wizards of the Coast
If your commander loses the choose a Background ability or stops being a Background during the game, as appropriate, it is still your commander.
No other card is named in this ruling.
#5June 10, 2022· Wizards of the Coast
Once the game begins, your two commanders are tracked separately. If you cast one, you won't have to pay an additional the first time you cast the other. A player loses the game after having been dealt 21 combat damage from any one of them, not from both of them combined (although your Background won't usually be a creature anyway).
No other card is named in this ruling.
#6June 10, 2022· Wizards of the Coast
If something refers to your commander while you have two commanders, it refers to one of them of your choice. If you are instructed to perform an action on your commander (e.g. put it from the command zone into your hand due to Command Beacon), you choose one of your commanders at the time the effect happens.
No other card is named in this ruling.
#7June 10, 2022· Wizards of the Coast
An effect that checks whether you control your commander is satisfied if you control one or both of your two commanders.
No other card is named in this ruling.
#8June 10, 2022· Wizards of the Coast
You can choose two commanders that are the same color or colors.
No other card is named in this ruling.
#9June 10, 2022· Wizards of the Coast
If a card refers to a commander creature you own, a Background won't usually be counted or included for that effect. If another spell or ability causes your Background to become a creature, however, it will be included. Any effect that refers to your commander or a commander you own or control without specifying creature will apply to a Background that is your commander, as appropriate.
No other card is named in this ruling.
#10June 10, 2022· Wizards of the Coast
If you control a Background that grants an ability to commander creatures you own, and you own more than one commander creature, each of them will have that ability.
No other card is named in this ruling.