Damn, my 10 points ): I had written 'including passives' before but it got removed in the editing process P:
Anyways, does this mean that everyone will be submitting pieces named assassin? Theme doesn't count so why not.
My impression of this guy is pretty good, though I think it is too strong and easily leads to annoying matchups. With the mobility, it basically means that any row the foe doesn't guard can just have this unit teleport to it, and in particular this is going to lead to a lot of early mates. Id score something like:

Inspires fear from across the field - 10 (Very very scary)
Breaks common chess intuition with a surprising move - 2 (Just a normal attack, even if it can be surprising)
Causes aggressive matches - 3 (You can be aggressive, but the opponent will be forced to play very defensively and it seems likely to lead to a lot of checks, in the endgame it gives rise to unstoppable checking wars which is unfun and slow)

I would put some kind of debuff to make infinitely checking disadvantageous and perhaps make it less mobile. Category 2 is going to be hard for you without magic or custom abilities.