You can find it in gen2 Rockwells inners. Yes he’s gut he’s heart. And you will find the summoner’s if you get in there spot light prepare to fight. It will summon all kind of stuff like reaper’s and other dangerous stuff. But focus on the summoner and then on it’s minion’s.

Hope you defeat it. Or you could use the easy way.

Step 1 go in creative mode. Step 2 take out you blink riffle and find the summoner. Step 3 kill it with kill mode.