If an opponent casts (or cascades into) Hypergenesis, starting with the caster each player takes it in turns to put a Creature, Artifact, Land or Enchantment onto the Battlefield. This can be repeated any number of times, and as a result potentially lead to some confusion.
1) I put an Emrakul, the Aeons Torn onto the battlefield, my opponent puts an Oblivion Ring onto the battlefield. Next, I put a Griselbrand onto the Battlefield, my opponent puts a Detention Sphere onto the battlefield. Finally, I put an Iona, Shield of Emeria onto the battlefield, My opponent puts an Island onto the battlefield. Can my opponent exile my Iona?