[@Leos Klien] Geas actually find its origin in English and Irish mythos, and is also featured as a D&D spell with a similar effect to the anime. The anime is definitely cool though. 

It can affect other players, yes. And that may indeed seem like very jarring. Do mind that we haven't assigned costs to the abilities yet. Geas in the first iteration of this RP had a relatively high cost associated with it as far as dark gods go. This is an ability that naturally has to be used sparingly. 
Naturally, my character only gives the order. Even though your character might do that against his/her will, it will still be in your character's hands to carry it out. Also, the consequences of having performed that action are still on your character's plate, and can lead to interesting roleplay opportunities. (Narcissa forced you to kill an important friend of yours, how does your character react?)
Lastly, I did build in the "no suicide" rule for obvious reasons, I very much agree that that would be the most anticlimactic death possible. 

The major abilities are meant to be powerful, and Geas is definitely high up there. But I don't think it'll become as problematic as you might believe it is.