Magus of the Unseen

{1}{U}, {T}: Untap target artifact an opponent controls and gain control of it until end of turn. It gains haste until end of turn. When you lose control of the artifact, tap it.

Masters Edition II

Fifth Edition

o1o
U, oc
T: Untap target artifact an opponent controls and gain control of it until end of turn. That artifact is unaffected by summoning sickness this turn. Tap the artifact if you lose control of it at end of this turn.

Ice Age

o1o
U,oc
T: Untap target artifact opponent controls and gain control of it until end of turn. If that artifact is an artifact creature, it can attack, and you may use any of its abilities that require oc
T as part of the activation cost. When you lose control of the artifact, tap it.

The artifact taps after returning to the opponent (if it is not already tapped), so any abilities triggered off it tapping happen at that time.

You may target an untapped artifact with this ability.

The artifact will gain haste whether or not it’s a creature. Of course, if it’s not a creature, haste will have no particular effect.

You tap the artifact when you lose control of it for any reason — because Magus of the Unseen’s effect ends, or because a spell or ability causes another player to gain control of it.