Onyxia and her lair are from Blizzard's Warcraft, Malefor's from Activision's Legend of Spyro and this screenshot's taken from Valve's Garry's Mod.

Malefor is so in love with a lovely dragon lady who is just as evil as he is that he'd flirtingly entertain her by puffing his belly up with his fiery breath and sings a serenade to her about what a mighty king and queen they shall be if they get married and would rule many worlds, not to mention prove how powerful they are by making their draconic stomachs so huge that not even this lair can contain it. Onyxia is so loving that song that she, too, inflates her own belly huge for him.