Tahngarth, First Mate — ruling 1

If Tahngarth gains menace, it can't be blocked at all.

Tahngarth, First Mate — ruling 2

Giving control of Tahngarth to an opponent doesn't cause it to become untapped. This means that Tahngarth may end up attacking several times between your turns.

Tahngarth, First Mate — ruling 3

If an opponent is attacking a planeswalker but not that planeswalker's controller, Tahngarth can't attack that player.

Tahngarth, First Mate — ruling 4

If an opponent attacks a player or planeswalker but each creature attacking that player or planeswalker is no longer attacking that player or planeswalker by the time Tahngarth's ability resolves (most likely because they've left the battlefield), that player or planeswalker can't be chosen for Tahngarth to attack. If there's no player or planeswalker to choose this way but you still choose to give the opponent control of Tahngarth, Tahngarth isn't attacking.

Tahngarth, First Mate — ruling 5

Players can't take any actions between the time you choose to give control of Tahngarth to the attacking player and the time it becomes an attacking creature.

Tahngarth, First Mate — ruling 6

Although Tahngarth is an attacking creature if it uses its triggered ability, it was never declared as an attacking creature. This means that abilities that trigger whenever a creature attacks won't trigger when it changes control and becomes attacking.

Tahngarth, First Mate — ruling 7

Any effects that say that Tahngarth can't attack or can't attack a certain player (such as that of Propaganda, or if Tahngarth gains defender) affect only the declaration of attackers. They won't stop Tahngarth from becoming an attacking creature through its triggered ability.

Tahngarth, First Mate — ruling 8

Tahngarth may end up attacking its owner this way.

Tahngarth, First Mate : règles, fonctionnement et interactions (MTG)

Tahngarth, First Mate 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

Tahngarth can't be blocked by more than one creature. Whenever an opponent attacks with one or more creatures, if Tahngarth is tapped, you may have that opponent gain control of Tahngarth until end of combat. If you do, choose a player or planeswalker that opponent is attacking. Tahngarth is attacking that player or planeswalker.

How it works : Tahngarth, First Mate

Original English text — Wizards of the Coast / Scryfall

#1August 23, 2019· Wizards of the Coast
If Tahngarth gains menace, it can't be blocked at all.
No other card is named in this ruling.
#2August 23, 2019· Wizards of the Coast
Giving control of Tahngarth to an opponent doesn't cause it to become untapped. This means that Tahngarth may end up attacking several times between your turns.
No other card is named in this ruling.
#3August 23, 2019· Wizards of the Coast
If an opponent is attacking a planeswalker but not that planeswalker's controller, Tahngarth can't attack that player.
No other card is named in this ruling.
#4August 23, 2019· Wizards of the Coast
If an opponent attacks a player or planeswalker but each creature attacking that player or planeswalker is no longer attacking that player or planeswalker by the time Tahngarth's ability resolves (most likely because they've left the battlefield), that player or planeswalker can't be chosen for Tahngarth to attack. If there's no player or planeswalker to choose this way but you still choose to give the opponent control of Tahngarth, Tahngarth isn't attacking.
No other card is named in this ruling.
#5August 23, 2019· Wizards of the Coast
Players can't take any actions between the time you choose to give control of Tahngarth to the attacking player and the time it becomes an attacking creature.
No other card is named in this ruling.
#6August 23, 2019· Wizards of the Coast
Although Tahngarth is an attacking creature if it uses its triggered ability, it was never declared as an attacking creature. This means that abilities that trigger whenever a creature attacks won't trigger when it changes control and becomes attacking.
No other card is named in this ruling.
#7August 23, 2019· Wizards of the Coast
Any effects that say that Tahngarth can't attack or can't attack a certain player (such as that of Propaganda, or if Tahngarth gains defender) affect only the declaration of attackers. They won't stop Tahngarth from becoming an attacking creature through its triggered ability.
No other card is named in this ruling.
#8August 23, 2019· Wizards of the Coast
Tahngarth may end up attacking its owner this way.
No other card is named in this ruling.
#9August 23, 2019· Wizards of the Coast
Commander damage is tracked for each commander. It doesn't matter who controls the commander as it deals damage. If Tahngarth deals 10 damage to one player while under your control and another 11 damage to that same player while under a third player's control, the player loses the game.
No other card is named in this ruling.