[Ronin]

Not too opinionated on the d20 vs unblockable issue - the unblockables route feels cleaner, but the d20 route gives you more dials to turn.

I don't think the d20 modifiers would be too much of a concern. It wouldn't be easy to modify his roll, since he won't be in clear sight of anyone or even on the battlefield when he uses the power. You couldn't boost it with Batman III, who's the d20-modifying elephant in the room.