Merry, Warden of Isengard — ruling 1

"Partner with [name]" represents two abilities. The first is a triggered ability: "When this permanent enters the battlefield, target player may search their library for a card named [name], reveal it, put it into their hand, then shuffle their library."

Merry, Warden of Isengard — ruling 2

Note that the target player searches their library (which may be affected by effects such as that of Stranglehold) and that the card they find is revealed, even though these words aren't included in the ability's reminder text.

Merry, Warden of Isengard — ruling 3

The second ability represented by the "partner with [name]" keyword modifies the rules for deck construction in the Commander variant and has no function outside of that variant. If a legendary creature card with "partner with [name]" is designated as your commander, the named legendary creature card can also be designated as your commander. For more information on the Commander variant, please visit Wizards.com/Commander.

Merry, Warden of Isengard — ruling 4

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 Frodo, Adventurous Hobbit and Sam, Loyal Attendant are your commanders, your deck may contain cards with white, black, and/or green in their color identity, but not blue or red.

Merry, Warden of Isengard — ruling 5

Both commanders start in the command zone, and the remaining 98 cards of your deck are shuffled to become your library.

Merry, Warden of Isengard — ruling 6

To have two commanders, both must have the partner ability or corresponding "partner with" abilities as the game begins. A creature with a "partner with" ability can't partner with any creature other than its designated partner. Losing a partner ability during the game doesn't cause either to cease to be your commander.

Merry, Warden of Isengard — ruling 7

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 damage from one of them, not from both of them combined. Command Beacon's effect puts one into your hand from the command zone, not both.

Merry, Warden of Isengard — ruling 8

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

Merry, Warden of Isengard : règles, fonctionnement et interactions (MTG)

Merry, Warden of Isengard 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

Partner with Pippin, Warden of Isengard (When this creature enters, target player may put Pippin into their hand from their library, then shuffle.) Whenever one or more artifacts you control enter, create a 1/1 white Soldier creature token with lifelink. This ability triggers only once each turn.

How it works : Merry, Warden of Isengard

Original English text — Wizards of the Coast / Scryfall

#1June 16, 2023· Wizards of the Coast
"Partner with [name]" represents two abilities. The first is a triggered ability: "When this permanent enters the battlefield, target player may search their library for a card named [name], reveal it, put it into their hand, then shuffle their library."
No other card is named in this ruling.
#2June 16, 2023· Wizards of the Coast
Note that the target player searches their library (which may be affected by effects such as that of Stranglehold) and that the card they find is revealed, even though these words aren't included in the ability's reminder text.
No other card is named in this ruling.
#3June 16, 2023· Wizards of the Coast
The second ability represented by the "partner with [name]" keyword modifies the rules for deck construction in the Commander variant and has no function outside of that variant. If a legendary creature card with "partner with [name]" is designated as your commander, the named legendary creature card can also be designated as your commander. For more information on the Commander variant, please visit Wizards.com/Commander.
No other card is named in this ruling.
#4June 16, 2023· 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 Frodo, Adventurous Hobbit and Sam, Loyal Attendant are your commanders, your deck may contain cards with white, black, and/or green in their color identity, but not blue or red.
No other card is named in this ruling.
#5June 16, 2023· Wizards of the Coast
Both commanders start in the command zone, and the remaining 98 cards of your deck are shuffled to become your library.
No other card is named in this ruling.
#6June 16, 2023· Wizards of the Coast
To have two commanders, both must have the partner ability or corresponding "partner with" abilities as the game begins. A creature with a "partner with" ability can't partner with any creature other than its designated partner. Losing a partner ability during the game doesn't cause either to cease to be your commander.
No other card is named in this ruling.
#7June 16, 2023· 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 one of them, not from both of them combined. Command Beacon's effect puts one into your hand from the command zone, not both.
No other card is named in this ruling.
#8June 16, 2023· 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.
#9June 16, 2023· Wizards of the Coast
The triggered ability of the "partner with" keyword still triggers in a Commander game. If your other commander has somehow ended up in your library, you can find it. You can also target another player, whether or not they have that card in their library.
No other card is named in this ruling.

Interactions with other cards

These cards' text mentions Merry, Warden of Isengard.

Pippin, Warden of Isengard

Partner with Merry, Warden of Isengard (When this creature enters, target player may put Merry into their hand from their library, then shuffle.) {1}, {T}: Create a Food token. {T}, Sacrifice four Foods: Other creatures you control get +3/+3 and gain haste until end of turn. Activate only as a sorcery.