Mashup of Control Magic and Ardent Militia.

Hm, what's vigilance on mind control? I'm not sure, but I went with "mind control that can block" (although since mind control can already block, I had to remove the control during your turn instead).

I'm not sure if this works as a card; it may be too confusing or too swingy.

Weird. Most conditional Mind Controls have still been (or at least two coloured mana) in the mana cost, though Vapor Snare is the exception. Given that the principal use of Mind Control is attacking with the creature, I think this could probably go a little cheaper; I'd say .

Having jumped straight into the nitty-gritty details, I should say this is a fascinating idea. It does work against letting games actually finish (in a way that vigilance doesn't), and a set doesn't want too many cards like that, but occasional ones are fine.

It's a nicely blue take on pacifism. Sightly annoying for the stealing of utility creatures; as opponents need to remember to tap them, the card won't prompt; and having to pass the card back and forth could get annoying.

I think I'd suggest you consider "At the end of its controllers turn, they choose an opponent to control enchanted creature." Which kinda does the same thing; but makes it silly in multiplayer rather than a lot better.

That's also red: Custody Battle :)

Good point about tapping though... in fact, hang on, does this let the creature actually ever untap at all? I think it does't.

Nope; hope you stole a creature they can't trivially tap :)

Thank you both! OK, I agree with the downsides of a mostly-defensive enchantment, but I can't remove it without removing the card entirely.

But at a minimum I need something that fixes the untapping problem. In fact, I quite like that you gain control of it during their turn, and vice versa (so in multiplayer you split the other turns according to turn order). But it would still need to say "untap it", and if it were "at the start of upkeep" it would let either of you use instant-speed abilities first which is a bit confusing.

Or maybe "at the end of each player's turn, they untap ~ and gain control of it"...? But it's feeling less blue by the moment :)

Adding "When you gain control of enchanted creature, untap it" to the end it fixes your problem and doesn't seem un-blue.

Or I could make it "whenever a player is attacked, they gain control of enchanted creature and untap it". That's more confusing, and further from "vigilance", but plays into the "militia" flavour...