“Only creatures you control on the battlefield when Arm with Aether resolves gain the ability. Creatures you gain control of later in the turn won’t be affected.
The affected creatures keep the ability even if another player gains control of them that turn. If such a creature deals damage to you, the ability will trigger, targeting a creature you control.
The granted ability triggers on all damage dealt by the creature to one of your opponents, not just combat damage.
In a multiplayer game, if an affected creature deals damage to two of your opponents simultaneously, the ability will trigger twice.”