History
The Collector was an Elder of the Universe. Like his fellow Elders, the Collector was a native of the previous universe, which predated the one where he would come to reside. This previous universe was destroyed by the Celestials, who overpopulated it.
The Elders began plotting to recreate the universe they had been born in. As the Celestials had also survived, the Elders knew not to oppose them directly, and instead sought to use the remains of the Celestials' method of procreation (imbuing planets with their essence and genetically manipulating its inhabitants) as the building blocks of their old universe. To that end, they created the Realm of the Dead, which would serve as a collecting pool for both the souls and the matter of the new universe. The Elder known as Death would be charged with stewardship over this realm.
The Elders' plot was ultimately foiled by Kree hero Mar-Vell, who killed Death and created Paradise as a shield around the universe, to protect it from the meddling of the Elders and the Celestials.[1][2]