Rift herald minions do pure damage. (or something)

I ulted as tryndamere and died mid ult while fighting an enemy graves that had rift herald buff. First time he had a huge wave of minions when I ulted and i died. The second time I fought him he still had the buff but no minions were near him and i did not die. I'll do more testing but it seems like what killed me were the minions.

And before people asked if i just messed up ult. No i did not forget to ult (it went on cooldown) and no i didn't ult too early.