Thanos decides to attack Asguard in hopes of finding secrets to the universe, while accidently opening a portal to Apokilips. Darkseid finds out about this portal and decides to check it out, and then he finds asgaurd. So now Thanos armys and Darkseid's army are in war on Asgaurd.
Darkseid and Thanos are at there fullest power! I don't believe this has been done before.