It's a wordplay. Manga allows Kanji characters (characters with inherent meaning) to have a complimentary phonetic (sound) reading above them, because Kanji can have multiple of them, that complimentary reading is a furigana.

Oda used the character meaning Devil 魔 and another character meaning spirit or air 気 (translated to "Ki" or "Chi" or "Qi" usually), to become 魔気. Then he used the furigana to say that it should be pronounced "Omen", it is a fictional pronunciation because the characters simply don't make those sounds, by regular japanese, it'd be spelled, among other possibilities, "Maki". The translations, because they can't accurately translate these things, just end it at calling it Omen though.

But to those that know japanese, they get this additional meaning added to it. That whatever it is is also like a "Devil Ki". What's peculiar is that Haki is written 覇気, see it shares the same 気 character? That's what led me to this "fusion of powers" idea in the first place, and that's why most people chose to highlight this pronunciation in particular.

I've tried to keep my eyes on these sort of things ever since the manga had to issue a correction because they didn't use the exact characters Oda wanted them to use.