I swear I have cleared Mourndale many time before, hunting for Grava and everything, but today I was completing Ultimate with my 6th character and suddenly out of nowhere this guy
I was like wtf, did they send a special boss just for the Spellbreaker, man, it’s scarry. Then I ofcourse killed him in 3 or 4 seconds, but damn, that was a scarry and a nice surprise, how did I not manage to catch him before. Also, I don’t recall him being in any of Crucible waves.
If it’s the same one I’m thinking of, there’s a spot in the middle(ish) of the map that has a body or bone pile that when looted spawns the boss in at least for me.
It has specific spawns in Mourndale. And i mean specific, like having to be really near where it can spawn.
I always thought he spawned from a chest, but if could also have been bodies or bone piles as you said.
I found him on each of my first clears of Mourndale, the other times I was there I did not care to loot everything, so I’m not sure if he’s a one off.
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For me, he always spawned at the chest in the south east, near the bridge at the edge of the map.
It is so freaking bizzare that I only met him on my 6th playthrough while everyone else seems to have one or more encounters with him 
There’s at least three or four possible locations for this guy, and he is, for me at least, always triggered by looting a resistance corpse.