The Amalgam Universe is a parallel universe created and destroyed as a result of a cross-Multiversal conflict between cosmic beings.
When two entities known as the Brothers (the embodiments of both the Marvel and DC multiverses) remembered each other's existence they pitted combatants from their respective realities together. The winner would then destroy the loser (and its own respective multiverse), to prevent this from happening, the Living Tribunal and the Spectre joined forces and combined the two universes together, creating the Amalgam universe. This universe had its own unique history which was a combination of both multiverses.
With the help of the hero named Access, the Living Tribunal and the Spectre managed to separate the two realities once more and the Amalgam Universe ceased to exist. Doctor Strangefate had managed to survive the destruction of the Amalgam universe and escaped into the Marvel Universe. There he took over the mind of Doctor Strange and attempted to recreate the Amalgam Universe by forcibly combining the two realities once more. He was stopped by Access before he could complete his goal. Doctor Strange managed to create a pocket dimension between the DC and Marvel Universe, and placed Doctor Strangefate and the entire Amalgam Universe inside it, restoring the lost reality and all its inhabitants.