When you find a reaper QUEEN make sure that you have a light pet of some sorts and turn it on. This will get rid of the 80% damage reduction. Next, attack it until the health is below 10% (until it starts glowing pink). Then turn off the light pet and approach her. When she starts sniffing you l, stay completely still and wait for her to impregnate you.

Make sure to get to an enclosed room and then wait for the reaper to pop out. You can gain exp for the reaper’s stats by killing wild creatures when you are still impregnated. Also make sure to eat a reaper pheromone gland to ensure that it doesn’t kill you. After all that, just raise it like you would a baby rex. Like!