Armix, Filigree Thrasher — ruling 1

Armix's ability goes on the stack without a target. While that ability is resolving, you may discard a card. If you do, a second ability triggers and you pick one target to get -X/-X. This is different from abilities that say "If you do . . ." in that players may cast spells and activate abilities after you've discarded a card but before the creature gets -X/-X.

Armix, Filigree Thrasher — ruling 2

While resolving Armix's ability, you can't discard more than one card to give more than one creature -X/-X.

Armix, Filigree Thrasher — ruling 3

The value of X is determined only as Armix's ability resolves. Once that happens, the value of X won't change later in the turn, even if the number of artifacts in your graveyard and under your control changes.

Armix, Filigree Thrasher — ruling 4

If the discarded card is an artifact card, it will be counted as you determine the value of X, provided it's still in the graveyard or, optimistically, on the battlefield under your control at that time.

Armix, Filigree Thrasher — ruling 5

If a creature is attacking a planeswalker, that planeswalker's controller is the defending player.

Armix, Filigree Thrasher — ruling 6

If your Commander deck has two commanders, you can only include cards whose own color identities are also found in your commanders' combined color identities. If Falthis and Kediss are your commanders, your deck may contain cards with black and/or red in their color identity, but not cards with green, white, or blue.

Armix, Filigree Thrasher — ruling 7

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.

Armix, Filigree Thrasher — ruling 8

To have two commanders, both must have the partner ability as the game begins. Losing the ability during the game doesn't cause either to cease to be your commander.

Armix, Filigree Thrasher : règles, fonctionnement et interactions (MTG)

Armix, Filigree Thrasher 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

Whenever Armix attacks, you may discard a card. When you do, target creature defending player controls gets -X/-X until end of turn, where X is the number of artifacts you control plus the number of artifact cards in your graveyard. Partner (You can have two commanders if both have partner.)

How it works : Armix, Filigree Thrasher

Original English text — Wizards of the Coast / Scryfall

#1November 10, 2020· Wizards of the Coast
Armix's ability goes on the stack without a target. While that ability is resolving, you may discard a card. If you do, a second ability triggers and you pick one target to get -X/-X. This is different from abilities that say "If you do . . ." in that players may cast spells and activate abilities after you've discarded a card but before the creature gets -X/-X.
No other card is named in this ruling.
#2November 10, 2020· Wizards of the Coast
While resolving Armix's ability, you can't discard more than one card to give more than one creature -X/-X.
No other card is named in this ruling.
#3November 10, 2020· Wizards of the Coast
The value of X is determined only as Armix's ability resolves. Once that happens, the value of X won't change later in the turn, even if the number of artifacts in your graveyard and under your control changes.
No other card is named in this ruling.
#4November 10, 2020· Wizards of the Coast
If the discarded card is an artifact card, it will be counted as you determine the value of X, provided it's still in the graveyard or, optimistically, on the battlefield under your control at that time.
No other card is named in this ruling.
#5November 10, 2020· Wizards of the Coast
If a creature is attacking a planeswalker, that planeswalker's controller is the defending player.
No other card is named in this ruling.
#6November 10, 2020· Wizards of the Coast
If your Commander deck has two commanders, you can only include cards whose own color identities are also found in your commanders' combined color identities. If Falthis and Kediss are your commanders, your deck may contain cards with black and/or red in their color identity, but not cards with green, white, or blue.
No other card is named in this ruling.
#7November 10, 2020· 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.
#8November 10, 2020· Wizards of the Coast
To have two commanders, both must have the partner ability as the game begins. Losing the ability during the game doesn't cause either to cease to be your commander.
No other card is named in this ruling.
#9November 10, 2020· 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 damage from any one of them, not from both of them combined.
No other card is named in this ruling.
#10November 10, 2020· 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.
#11November 10, 2020· 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.
#12November 10, 2020· Wizards of the Coast
You can choose two commanders with partner that are the same color or colors. In Commander Draft, you can even choose two of the same commander with partner if you drafted them. If you do this, make sure you keep the number of times you've cast each from the command zone clear for "commander tax" purposes.
No other card is named in this ruling.