Cthulhu is a God, well, an Old One , to exact. In H.P Lovecraft Stories involving him, we is sleeping under the ocean, and when he awakes,  shall cause the End of Earth.He is mega-OP, with one billion Hearts, and will only be encountered on in level 800+ oceans, under exact conditions. The conditions are:

1. You must be in an ocean for at least one minecraft day

2. You must be at least level 200

3.You must Stop at an island, and rest for one day there

4. You must have at least 200+ fortitude, and 150+ strength


If you are stupid enough to obey the following conditions you will have to go against Cthulhu. He has the following powers:

1. Destroy blocks: Like the Ender Dragon, But at a much larger radius.

2.Insanity: Looking at Cthulhu is dangerous, per tick, there is a one in 1000 chance you get the following effects: 

Blindness-8 min, poison-10 seconds and nausea-3 minutes