Links to the other crossovers would be appreciated so that I don't have you repeating something you know has already been done.
I've played Overlord, the expansion pack, Overlord 2, and Overlord: Dark Legends as well as The Elder Scrolls Oblivion and Skyrim, so hopefully I can be of help. To clarify the nomenclature, the original overlord is called the "Olderlord," the PC from Overlord is of course "Overlord," the PC from Overlord 2 is the "Underlord" because his "tower" is underground, and the PC from Overlord: Dark Legends is the "Overlad."
The first question is whether this is going to be a Trollestia universe. There have been fics reimagining Princess Celestia as a tyrant, or at least abusing her powers, which will in turn have an effect on the new overlord. I'm going to assume the answer is no, but I'd like you to at least consider Luna/Nightmare Moon, Discord, and especially King Sombra's role in your fic.
Second question is "purple minions," the hypothetical new variety of minions created when the lifeforce of one of the pony races is used instead of sheep. I would like you to at least consider a race of flying minions created from a former pegasi as among the reason for the invasion. I would also like consideration to be given to "black minions" created from Changelings.
Now a moment of headcanon: The stereotypical evil overlord was defeated by a party of eight fairly standard heroes: a halfling bard, an elf ranger, a dwarf warrior, a human paladin, a human thief, a half-giant fighter, a wizard, and a martyr. The name, race, and character class of the martyr who died in the act of defeating the evil overlord were never revealed, but we assume it to be a cleric of the Mumbler's religion. The original overlord, the Olderlord, cheated death by invading the mind of the old wizard. The martyr was resurrected by the few remaining evil minions in the broken tower to act as their temporary leade