Consider the debuff Brittle - 30% longer cc duration and 12 - 20.5% of maximum HP when immobilized. Now consider that his ultimate, though not too easy to land applies it on both hits. Now though brittle doesn't stack, a champion who gets hit by both parts is more or less marked to lose 41% of their maximum HP.
Allies that effectively wombo combo with Ornn's W and R alone at level 18 yield 77.5% of targets' maximum HP from Ornn's kit alone
(R1 applies brittle, R2 procs and reapplies brittle, [ally AoE immobilize], while CC'd Ornn walks up and W's, [another ally AoE immobilize])
Ornn can also do the same but requires 2 autos