So if from what i learned of G1 Tirek his big weapon was the Rainbow of Darkness. He wanted to use it to bring eternal night to Equestria. So since Nightmare Moon did the whole Eternal Night shtick if G4 ever had it's own Rainbow of Darkness what do you think it should do. Well keep the whole turning Ponies into Dragon Demon things from G1 since that's cool. But it should have a least another major power to it as well.

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This is just a thought-but, what if Tirek did have a Rainbow of Darkness in the past before being defeat and imprisoned-and he used it for that purpose, but he somehow absorbed it into his body like Rainbow Power and that's what allows him to steal magic?

There's actually a rather excellent Fanfic that involves the Rainbow of Darkness as a plot point.
Tirek controlling it is even referenced at a few points.

(Granted, it's post-Season 4, and it's a crossover with Team Fortress 2, but it's still awesome.)