I told yall I would make ponies for everyone, now it's Eclipse's turn.

Yes, his armor is a mix of Nightmare Moon and Daybreaker, his name is Eclipse, it's the most fitting thing imaginable.

My idea of how Eclipse was "born" revolves around Luna and Nightmare Moon, where MA is jealous of his brother getting all of the attention with his sunlight, but instead of eternal night, he uses dark magic to steal Mandara's magic and transforms into Eclipse, controller of both the sun and the moon.

I'd think that Mandoria would fight Eclipse since Mandara has no magic, he then somehow pulls the corrupted magic from MA and returns most of Mandara's magic.  The rest of the corrupted magic gets locked away somewhere, but then released after a long time and becomes a sentient being.  It reinfects MA and becomes his MS-PD demon, then eventually reforms and gets his own body because this is MLP status quo.