ppaladin123
Adventurer
Why as an immediate reaction? Just make it a free action. Making it an immediate reaction is a pretty big cost (especially at higher tiers, when the assassin arguably struggles the most) for something that the assassin really shouldn't be losing in the first place at all.
I was just going with the precedent set by the feat. I doubt it would be a problem to change it to a free action like you said though.