I throw Ainz into the world of Code Geass (the anime), while technically a crossover, I am an Ainz over Lelouch kind of guy, so the story is more about Ainz being better and cool, so I stuck it here :3

This is the first part of the story of the 10,000 year reign of the Overlord Ainz Ooal Gown.

It's funny how a little change in events can impact everything after. And sometimes, it become just that much harder for everyone. But even if it's a bumpy road getting out, and more importantly, gaining her forgiveness, nothing's going to stand between Wheatley and redemption. Human!Wheatley. Originally posted on AO3. Will heavily include Portal Stories: Mel, uh, bits

Without Geass, but with his friends and comrades that will help him on his path, Lelouch Vi Britannia sets out to liberate the Japanese people on his own terms, rallying those around him under the pen, rather than the blade. Nonviolent!AU, with Lelouch and Suzaku working together.

After breaking the Barrier, Frisk knew there was one soul that needed to be freed, going deep inside himself, he frees the girl trapped in his soul. But instead of thanking him, she tries to kill him! It's up to Frisk and friends to show her Love is not so bad after all (Completed)

The first few years everything was alright. Nagisa was leading a happy life, he was a teacher, had friends, his family was coming back together. What could possibly go wrong? Then came the nightmares, the treacherous thoughts. He starts spiraling into depression and no matter how much Akari tries, not even with Ritsu's infinite well of knowledge, can she pull him back. Time-Travel

When Lelouch and Nunnally joined the Kozuki Resistance, Things had changed for the Resistance group. they began to follow Lelouch's principles. with the power of Geass, Allies and friends, and the Ganymede, The Black Knights became a powerful force to be reckoned with.

Frisk was living on the surface with their new family. Everything was perfect except, the dreams that seem to belong to someone else. Something is left undone. The past is pulling them back. The world is reset, everything is undone. Somehow, they have to break the cycle and save everyone, and themselves. (pacifist run spoilers)

While She was dead, She had unknowingly been robbed. Years later, it's up to the most unlikely mix of robots and humans that ever existed to save the world and reclaim what has been stolen – one step at a time. Post-Portal 2 AU

Follow Lelouch vi Britannia's rise from the beginning. Marianne does not die, and Japan is not invade in 2010. It starts a little slow, but really gets rolling after chapter 15.