River said:
You assume this rogue has no Charisma. Most rogues do. Potentially a rogue with 18 Charisma could pull the trick 7 times a day!
You're right. But I still don't see it as imbalanced in any way. Let's face it, this ability allows a rogue to get one sneak attack against one target. In a normal party, a rogue is going to have plenty of opportunities to flank and sneak attack anyway, so the advantage of being able to sneak attack a limited number of times without someone to flank, while nice, is hardly game-breaking. Also, the short duration nullifies nearly any other use of being invisible
except for a sneak attack, since out of combat it lasts less than six seconds.
River: this ability is
never better than
Improved Invisibility. Since you reappear each round, your opponents are always going to have a decent idea of where you are, at least within 30', so area effect spells are still a go. Also, because you reappear, you are either going to tell them
exactly where you are (if you cloak, then use it to get a sneak attack,) or they're going to get an AoO against you (if you attack, then use it after and try to move away.)
Keep in mind as well that once
Improved Invisibility is cast, the rogue loses no time maintaining the spell. But this spellnote requires a ME action for each disappearance, limiting actions and mobility.