Haldan, Avid Arcanist — ruling 1

Haldan's ability applies only while it's on the battlefield. You'll be able to play cards you exiled with fetch counters before Haldan entered the battlefield, and if Haldan leaves, you won't be able to play cards that were exiled with fetch counters until Haldan returns.

Haldan, Avid Arcanist — ruling 2

Haldan doesn't change when you can play the exiled cards. For example, if you exile a sorcery card, you can cast it only during your main phase when the stack is empty. If you exile a land card, you can play it only during your main phase and only if you have an available land play remaining.

Haldan, Avid Arcanist — ruling 3

A creature card that you exiled with a fetch counter on it may be cast as a noncreature spell if a rule or effect (such as that of adventurer cards) allows you to do so.

Haldan, Avid Arcanist — ruling 4

In a multiplayer game, if a player leaves the game, all cards that player owns leave as well. If you leave the game, any spells or permanents you control that Pako fetched are exiled.

Haldan, Avid Arcanist — ruling 5

"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."

Haldan, Avid Arcanist — ruling 6

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.

Haldan, Avid Arcanist — ruling 7

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.

Haldan, Avid Arcanist — ruling 8

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 Haldan and Pako are your commanders, your deck may contain cards with blue, red, and/or green in their color identity, but not cards with white or black.

Haldan, Avid Arcanist : règles, fonctionnement et interactions (MTG)

Haldan, Avid Arcanist 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 Pako, Arcane Retriever (When this creature enters, target player may put Pako into their hand from their library, then shuffle.) You may play lands and cast noncreature spells from among cards you exiled that have fetch counters on them, and you may spend mana as though it were mana of any color to cast those spells.

How it works : Haldan, Avid Arcanist

Original English text — Wizards of the Coast / Scryfall

#1April 17, 2020· Wizards of the Coast
Haldan's ability applies only while it's on the battlefield. You'll be able to play cards you exiled with fetch counters before Haldan entered the battlefield, and if Haldan leaves, you won't be able to play cards that were exiled with fetch counters until Haldan returns.
No other card is named in this ruling.
#2April 17, 2020· Wizards of the Coast
Haldan doesn't change when you can play the exiled cards. For example, if you exile a sorcery card, you can cast it only during your main phase when the stack is empty. If you exile a land card, you can play it only during your main phase and only if you have an available land play remaining.
No other card is named in this ruling.
#3April 17, 2020· Wizards of the Coast
A creature card that you exiled with a fetch counter on it may be cast as a noncreature spell if a rule or effect (such as that of adventurer cards) allows you to do so.
No other card is named in this ruling.
#4April 17, 2020· Wizards of the Coast
In a multiplayer game, if a player leaves the game, all cards that player owns leave as well. If you leave the game, any spells or permanents you control that Pako fetched are exiled.
No other card is named in this ruling.
#5April 17, 2020· 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.
#6April 17, 2020· 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.
#7April 17, 2020· 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.
No other card is named in this ruling.
#8April 17, 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 Haldan and Pako are your commanders, your deck may contain cards with blue, red, and/or green in their color identity, but not cards with white or black.
No other card is named in this ruling.
#9April 17, 2020· 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.
#10April 17, 2020· Wizards of the Coast
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.
#11April 17, 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 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.
#12April 17, 2020· 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 who might have that card in their library.
No other card is named in this ruling.

Interactions with other cards

These cards' text mentions Haldan, Avid Arcanist.

Pako, Arcane Retriever

Partner with Haldan, Avid Arcanist Haste Whenever Pako attacks, exile the top card of each player's library and put a fetch counter on each of them. Put a +1/+1 counter on Pako for each noncreature card exiled this way.