The creation of a realm leaves a void in the multiverse. These voids collectively form The Abyss. The forces of The Abyss seek to restore the harmony of the multiverse by merging themselves with their counter realms, annihilating both in a process called culmination.
Out of each abyssal void spawns an abysslord opposed to realm and realmlord alike. For example, a realm of death with a realmlord of life may spawn an abysslord of undeath, a mockery of both death and life.
The abysslords are most active when their respective realmlords are mortal. Should an abysslord defeat the realmlord they oppose and usurp them, that corresponding realm will fall back into the void, fulfilling the abysslord's purpose... at the cost of its own existence.