Garuda.Aurilius said:
Amnesia would be just a little game breaking. Especially on mobs that have notoriously bad TP moves.
It would depend on the implementation. I wouldn't assume it to be a 100% stick rate. Not like Silence is. Some (at least) NMs would likely be immune too, just like with Silence.
In some ways it'd be more feasible than, say, Break that actually works like the mob version, or Terror, or even Curse, though all of those would be welcome/interesting additions as well.
Powerful? Certainly. I should hope that very high level, job-defining spells would be. The other thing to figure is that it wouldn't be in a vacuum. RDM might get some great new enfeebles + Haste/Refresh II. Other mages would get similarly splendiferous new toys, and so on.