First time I stumbled on this rift today, it have zero trash mobs. It was only boss monsters. 1 at a time and it took 4 of them maybe 5 to spawn the final boss, but wow that was one of the best rifts. Loved it, someone told me they are a rare spawn rift but you can fish for it by opening and closing them over and over, while tempting as that might be. I dont like wasting time and that seems like a time waste but still was fun zipping thru a rift in under 45 seconds