Bittersweet Ending: The Overlord successfully saves the world from the Forgotten God but is separated from his mistress Rose and his unborn son. All of his accomplishments will also be undone by the Glorious Empire's Emperor.
Hijacked by Jesus: The game's backstory is actually hijacked from Wicca with the Consort turned into Satan. A sort of oddball metamoment that may pass audiences by.
Infinity–1 Sword: Five of them, one for each category: Armour, helmet, sword, axe and mace. They're all very nice, but especially in the case of the armor and helmet, they're unlikely to compete with fully upgraded Arcanium equipment.
Ironic Hell: All of the Abyss-levels are ironic hells, targeted at the races in general, and the heroes you killed in particular. So the Gluttonous Halflings ends up in a hell of exploding sheep and killer pumpkins, the lecherous Paladin ends up in Lady Land, etc.
Karmic Death: Subverted. The Overlord's death actually makes him ruler of Hell.
Large Ham: MY NAME WILL BE WHISPERED! MY NAME WILL BE SCREAMED!
Like a Badass out of Hell: Upon defeating the Bonus Boss of the Infernal Abyss, the denizens of that fiery pit swears allegiance to you - you have successfully taken over hell! Which is probably a good thing, considering that you're STUCK there.
No Name Given: We never do learn the Forgotten God's name, which kinda is the point since Mother Goddess cursed him so that no-one could remember him, thus removing source of power.
Planet Heck: The expansion adds several "Abyss" levels for you to explore and conquer, including a Bonus Boss at the end.
Schmuck Bait: Every realm in Overlord let's you kill sheep and smash containers without restraint. Try that in the Abyss, why don't you?