Amalgam Comics was a publishing imprint of both Marvel and DC. It contained fusions of two characters, creating a new one. Examples include, Super Soldier (Superman and Captain America), Thanoseid (Thanos and Darkseid), Doctor Doomsday (Dr Doom and Doomsday), etc. They also made a miniseries called Marvel vs DC.
The Amalgam universe was created when two entities created by the Living Tribunal, the Brothers, became aware of each other after eons of slumber in their respective universes. As the Brothers' rivalry would end up destroying their universes, residents of the Marvel and DC universes were forced to battle each other for survival. It ultimately concluded when both realities were saved from destruction by being merged into one new universe. Only the meta-mutant known as Doctor Strangefate knew the truth of his reality and attempted to stop the multiversal protector Access. But Access recruited Batman and Captain America aid him in undoing the merger and then convince the Brothers to cease their rivalry. Despite Strangefate's last attempt to recreate his reality, he is allowed to exist in a pocket universe created in his reality's image.
Power of this Verse
This verse is extremely powerful as it has The Brothers Yin and Yang (The Amalgam Brothers) who dwarfed even The Spectre and The Living Tribunal before being retconned into "mere" Universe level+ entities.