Kesh
First Post
I'd just make it a feat and roleplay the rest. The "bat" has his stats for Perception checks, and occasionally makes "suggestions" to him. The "suggestions" can A) simply be role-played; B) treated as a Social encounter (4 successes before 2 failures, or something like that) as long as they don't come up too often; C) impose Combat Advantage against him until he either succeeds at a saving throw to resist those evil ideas or acts on them.
A is probably the easiest, as there's little mechanical advantage to the "bat" outside the spying. Some players may feel gypped, but I think it's worth a feat as long as you restrict the speed of the familiar and make it noticable to enemies. I'd treat it like a minion: any hit takes it out, but he can spend a standard action to bring it back. It's not useful in combat, really.
A is probably the easiest, as there's little mechanical advantage to the "bat" outside the spying. Some players may feel gypped, but I think it's worth a feat as long as you restrict the speed of the familiar and make it noticable to enemies. I'd treat it like a minion: any hit takes it out, but he can spend a standard action to bring it back. It's not useful in combat, really.