Bionicle/High School DxD Crossover Challenge.
Why not?
1. Makuta Teridax will thought to have died in the final battle against Mata Nui on Bara Magna, but somehow, he ended up in the DxD Verse by some means.
2. Teridax will try to get allies through manipulation. Of course, he'll be talking to them through his shadow form.
3. He will formulate schemes to take over the DxD Verse. If you want, you can make up a story that God of the Bible didn't die, but just asleep. Imagine if Teridax took over his physical form and sent His consciousness far away from Earth like how he took over Mata Nui's original gigantic robotic body and sent his consciousness far away from the Matoran Universe along with the Ignika.
4. If you remember him saying "I have no allies, for I have no equals", then it's obvious he will betray his own allies.
5. It's up to you if you want Makuta Teridax to win or not.
Enjoy, and have fun writing.