It is just a normal assasin but:
-Is faster (because he is mounted)
-After he kills somebody, he becomes visible for 5 turns (so time to escape)
What do you think?
Random guy: "Lets use goblins." Me: "GOBLINS? THIS IS LAWRENCIAAAAA!"
Goodness no. Just no. Assassins are about as balanced as possible right now. Any more speed, and they become highly overpowered, especially since they can knock out castles.