Alright, hear me out:
Within the Resonant Fury Greaves' (VoTD Titan Legs') loretab, Xita states the following regarding Rhulk:
[i]"[He claimed to desire our powers for his Witness. But I overheard eons of their discussions. His omniscient god always reduced him to that which he claimed—[b]wrath.[/b]]"[/i]
[url=https://www.ishtar-collective.net/entries/resonant-fury-greaves?highlight=Resonant+Fury+Greaves]Source: Ishtar Collective - Resonant Fury Greaves[/url]
Furthermore, the flavor text for Nezarec's supposed glaive gives the following quote:
[i]"Rise, Disciple, and bear this gift with [b]pride[/b]." —Rhulk, Disciple of the Witness[/i]
[url=https://www.ishtar-collective.net/entries/nezarecs-whisper?highlight=Nezarec%27s+Whisper]Source: Ishtar Collective - Nezarec's Whisper[/url]
Given these two, alongside the notion of Nezarec's first ever mention being that of the Nezarec's [b]Sin[/b] helmet, perhaps this could be alluding to all of The Witness' disciples being derived from the seven cardinal sins? If you think about it, Calus could very well fill in for "Greed" given the overall theme surrounding him, and maybe he'll be used to represent that aspect in some manner throughout the course of Lightfall's story as it pertains to Neomuna?
One other quick side-theory I'd like to share in regards to this as well: Given that Osiris downed a cup of Nezquick before shoving a little man-on-man fan service down our throats to compliment the travesty that was...whatever that seasonal ending was supposed to be in the eyes of Bungie, perhaps a little bit of Nezarec's "Pride" aspect rubbed off on him?
I'm sure most of you reading this have already seen Byf's analysis on us potentially conjuring Xivu Arath's entire army via the waging of our war just as what had occured over Torobatl, and given that Xivu Arath--a constant of War--can be wrought forth through said constant...who's to say that Nezarec can't be wrought forth in the same kind of manner? Maybe Osiris will get a little ahead of himself after finally proving the existence of Neomuna to everyone?
Look, there's no way a Disciple of The Witness & one of D2's oldest mysteries is going to be concluded at some paracausal teabending nonsense. Something's gonna come up during the course of Lightfall and/or it's accompanying seasons, and so help me, if Bungie isn't going to have us turn Nezarec into a shiny new fushion rifle by the end of this light and dark saga, I'll chop it up to Curse of Osiris rising back up from it's fecal-laden grave to deliver one last kick to the collective balls of our entire community!...
...Look, I just really want another Rhulk, okay? Nobody can argue that he isn't a peak of Destiny storytelling.
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Personally I think, with this theory, that Gluttony suits Calus better. For a long time his main catchphrase was, “Grow fat from strength”, and he is a hard partier, and was known for holding many celebrations back on Torabatl. Sloth could possibly work too, since he just wanted to sit back and watch while the Darkness invaded. However, I feel as though gluttony suits him better.