Tahngarth, First Mate

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.

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

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

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.

Tahngarth may end up attacking its owner this way.

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.