This is an absolute shoo-in for the red common "negative" Geass slot.

See Geass to Die for discussion of the Geass type and mechanic.

Wonder whether this should be instant speed, to allow for use to create a surprise blocker. That's certainly in-flavour for how Lelouch uses his Geass, and I still think the set does want a higher-than-normal level of instants, so it's possible; but I probably want to make sure there are still some sorceries.

FWIW, I think the flavour would be really good as an instant (but I always have to fight the tendency to make everything instant).

Compromise, make it an enchantment with flash. (Geass really should be enchantments, shouldn't they?)

Also really interesting, as this is a safe way to protect your own creature, untap it, haste it, +1 it... ok, 4 mana and you give up the chance to use an opponents creature; but it's a nice option to have on the table.

Ah, that's true - I was imagining people using some of the instant speed somewhat-negative Geasses on their own creatures in response to nastier ones (e.g. Geass to Forget in response to Geass to Die), but this is nice in that it can be used on your own stuff completely without penalty (except for passing up the chance to use it on opponents' stuff). That may be enough to swing me to make it instant.

I was wondering if geasses could all be auras with "Enchanted creature can't be the target of other Geass spells". (Or maybe even give up the flavour and make it "other auras" so it's less parasitic.)

I think the downside of auras is that it makes one-turn effects (and even more so, removal effects) look very odd. But the downside of instants/sorceries is that you need some way to remember "geassness". I'm not sure which is better.

One way to go would have geasses exile, so you can tuck them under to indicate geassiness.

But auras are far simpler.

This is true, you do at the moment need to put a Geass counter on the creature, or something similar. Unfortunately making everything an Aura would be pretty odd as well, because it'd leave a bunch of ETB-effect auras just sitting around doing nothing... unless I guess I made them all also have some kind of ongoing effect. Hmm.

That would mean this one would have to turn blue, because it'd need to turn into permanent gaining control. (It would also have to become uncommon rather than common.)

FWIW, for this name, I prefer temporary instant-speed stealing because it seems to capture the feeling of "betrayal" a lot better to have it happen at the worst possible moment, more than have it be forever. (But that's not to say the common red geass has to be this card, if uncommon blue stealing is a better card you can come up with appropriate names and replacements.)

Oh! Inspired by V's last comment, could you make "exile under this card" an official part of the geass rules like a simpler haunt (on instants/sorceries only or an all geass cards). Something like "Exile this under its target. That gains protection from..." You'd need to specify whether the two effects are tied together or just usually go together (like the different implementations of flood counters). And make sure the edge cases were specified in the comp rules and obvious enough they didn't need reminder text. But it might be a compromise?