Trapping players isn't a good idea imo, would be an I win button if it was a warrior and a sin against a wiz/tao.
I assume the invisibility will last for a short time and have a fairly long cooldown. 3x damage is REALLY over the top i think, picture this, Assasin invisible in mass fight, wiz met/blizz assassin walks up bang 3x damage bang bang dead.
The multiplier is a good idea, just think 3x is way OTT, if possible code it so there is like 50% of 1.5x 40% 2x 10% 2.5x. Would need to put a big delay on it as I said before otherwise sins will just go invisible and knock the **** out of people.
With invisible spell you should code it like how clear ring is with mobs. Depending on your level compared to their level their is a %chance of their invisiblity working on you. Say they're level 40, you're level 40 there would be 75% chance, sin is level 40 your level 30, 95% chance (don't think it should ever be 100%). Sin is level 40 your level 60 10% chance, something like that.
From what i'm reading this looks like it is going well, good luck
/Nymph