Go to Hall of the Reaper Queen, thats where all reaper queens hang out. Build a trap there and shoot the reaper and turn the light off. If you see red aura, get off your mount and it will impregnate you.
I heard that these guys were based off of the Xenomorphs from the Alien franchise, and it definitely makes sense. Get impregnated by a queen, have a horrifying chestburster emerge and try to kill you? Seems pretty familiar!
Since i cant find this anywhere. Reapers take reduced damage naturally so its a good soaker. Dont bring light pets near theese because they take more damage and deal less damage.The reaper burns when light is near it. Nameless will attack it tho. Sorry for bad english.