Doomsday was a Kryptonian evolution experiment. He was grown by scientists then put on a planet and devoured by creatures. Then the scientists would recover the remains, make a new Doomsday, and throw him back out to the creatures. Eventually, he evolved to where he was stronger and tougher to the point that they locked him away and jettisoned him into space. He landed on Earth and was buried. He broke out and wrecked **** until Superman killed him. Basically, when he fights something he grows stronger to adapt to whatever may kill him like growing more bony protrusions and gaining strength. If he ends up getting killed, he comes back tougher to kill than the last time.