The Boss ran past me with it’s jumping move and landed off the ledge. I assume there was boss loot but nothing dropped since the bosses body was off the map.
That’s just awesome 
No guaranteed drop, but he does drop loot. It’s not spectacular unless you get his weapon (not guaranteed).
I would suggest to continue into sacrament, and potentially start a second realm to kill him again if you specifically want his weapon (and/or a real kill)
In sacrament you can get new weapons and enchant them, potentially making them very powerful