They come into 5e with Rifts Mega Damage Capacity stats.
"oh, I'm sorry, you thought Cthulhu's claw attack did TWELVE damage? No, that's 1200 damage, and make a save vs. liquification."
I was thinking more like Rolemaster: "Cthulhu hits you for 24 and a D critical, which breaks your arm and gives you -25 to everything until it's healed."