Calus was a literal reference to the King in Yellow
It's not a secret that Destiny lore heavily pushes itself into Lovecraftian themes for its aesthetics, It's themes, and all of its villain characters. They also use occult references fairly often to drive home the 'otherness' of a lot of our true foes.
See the Hive pantheon, The Witness, or anything like the scorn, or even the Winnower.
The King in Yellow is a short story collection, in the vein of weird-fiction and horror, written in the early 1900s by Robert w. Chambers.
There is no literal king to connect any of the stories, but rather a play literally called The King in Yellow.
It is an opera, whose second act is filled with so many beautifully horrible truths, that it is incomprehensible to a mortal mind. Parts of it can literally trap your thoughts and dreams. (Sleeping dreams, not ambitious dreams)
The play references the King and a city called Carcosa. It's not entirely clear if they're the king OF Carcosa, or a king visiting.
The King is clothed in tattered yellow cloths, and what appears to be a mask. Not much else is said.
Carcosa is specifically called out for being 'strange'. It has at least 2 moons. Shadows behave strangely inside the city.
The King in Yellow (this gets meta) within the opera, INSIDE the literal physical collection of stories, is meant to be something besides a king. It is implied that the King is a vessel, willing or otherwise, and that he is there not to conquer or attack or befriend, but to warn.
The King in Yellow is a Herald of doom to come. Something unknown has sent him to tell us that the end is nigh.
There's a few things that give it away to me personally. This is all my interpretation.
Emperor Calus- emperor is just another word for ruler, or king
Calus is surrounded by gold and decadence, convinced that the universe is on its way out. Gold isn't tattered yellow cloth, but it's close enough for me. Parts of Calus literally become gold.
After Calus met 'the voice in the dark' he began calling himself it's Herald. Because of this Voice telling him so, he believed that the end was nigh, and that his people should go out partying and that he would make it his purpose to inform others of their impending doom.
The leviathan: On top of the Leviathan is Calus' strange golden city.
Calus' mobile city doesn't have a name that I can find, but it does have at least 2 SUNS
These are, again opinion, however I don't think it's that much of a stretch to imagine that Bungie pulled inspiration from the weird fiction genre in general, rather than solely doing lovecraftian jams forever.
They even dug deep into occult themes, what the Witness is has a name; an egregore. It's literally a spirit or entity that arises from collective will and conviction of a group.
Thanks for reading! Thoughts?