Board Thread:New on Terraria Wiki/@comment-74.101.126.88-20121218114129/@comment-5033526-20130421221650

The Pathogen wrote: He does have a random chance to spawn. Check the Eye of Cthulhu page for more info.

You got that right.I went to fight Skeletron and was doing great,until the EoC spawned right in the middle of the battle! I got annihilated.