#bc of synopsis

As Lujanne explains in 2x02, the Moon arcanum is understanding the true nature of the relationship between appearances and reality, and we can only understand the appearance itself. This feels like a very fitting idea to come back to with Aaravos, who ostensibly never lies but routinely withholds or presents not entirely correct information. "How may I serve you?" when you're just going to be a pawn. Not telling a mourning Claudia that he was indeed the one who killed Viren so that she'll continue to do his bidding, with Claudia asserting that Aaravos "didn't lie" about the ritual in 6x01, and he didn't. We also see him wield the truth as a weapon with people like Khessa ("would you like to know the truth of her fate before you meet yours?") and Sol Regem (more on that here.)

Everything that he says is truth to him, and then he lifts it up as being objectively true (i.e. you're destined to play into my hands) even when it isn't necessarily true. We can also see Callum veering into mindsets that Claudia and Viren have had, where he believes he's past the point of saving ("I'm ruined, it's too late for me" "Promise me you'll kill me") or removing his own agency by admission ("Finnegrin was going to kill you, I didn't have a choice" / "Every step forward is a choice").

Callum understanding Aaravos' or others' actual truths versus their lies and the ones he's believing could be very fitting in S6, especially if he might be learning more about the existence of the Cosmic Council and who made their world the way it is. I think his existence may help lead to that "slow spiral of chaos" but that it won't be just or even Bad at all the way they'd feared, etc.

Claudia is also linked to lies and truth. She lies to others, but Aaravos notes that "If you tell the truth you will lose her," and she goes looking for her own deep truth in S6, but doesn't seem to fully find it. Terry asks her "What do you need to find your one deep truth?" and Claudia says that she needs her dad, but she and her mother have also made it clear that she "needed to stay with Soren" and her family (vs Viren telling her to pick the egg over Soren). With Viren gone and Aaravos manipulating her, Soren could easily be one of her guiding lights next season or in future seasons.

The Pearl

The moon is analogous, framing/appearance wise, to the prison. This is alluded to in 5x09 through framing, and then made even more direct in 6x09: Aaravos escapes his prison thanks to Claudia and (unbeknownst to her) she has become the prisoner, much like how Callum may physically free himself from chains in 5x08 but magically/emotionally chains himself further to Aaravos, or Viren shouting while in chains that he's finally free of the dark puppetmaster.

Basically, when Callum says in 6x01 that he's inside the pearl, I don't think (as of S6 / probably first half of S7 at least) that he's ever gotten out of it. Aaravos uses him even after his nightmare, and we know thanks to the pawn intro that Aaravos' final machinations for Callum also haven't yet come to play. I've been wondering if the Aaravos intro is going to change in S7, since he's out of his prison — and it still may — but if we look at it from the angle of Callum and Claudia both being stuck within the moon/pearl rather than just Aaravos himself, maybe it could stay.

And if Callum is stuck inside the moon-pearl, shattering it by understanding the arcanum ("the whole world is like a giant primal stone, and we're inside of it, and it's also in us") and/or with Claudia moving to the opposite of the moon could be useful, especially since Karim is a corrupted sun in his own way. Speaking of which let's talk about

Light and Dark

We know thanks to Claudia that black and white, or light and dark, are not always clear cut. Her hair thanks to her dark magic use changes like phases of the moon, with the light being bad and the dark being good. Conversely, her path is a dark one with the path of truth and light being withheld from her. In a similar manner, we see Callum's light (Rayla) being what led him into dark magic use and what led him out of it, and will likely see this pattern play out again. Aaravos is a representation of a path of darkness, but we also see the cube flashing a bright light in the pawn intro, tying light and darkness together for him as well.

Callum's understanding of himself that he gained through the Ocean arcanum can not simply become untrue or disappear, so I think recognizing that darkness isn't all he is ("I'm ruined" / "your heart's not full of darkness" "Neither is yours") even if there are dark parts of him and of his life would be fitting. It also seems that could be helpful with Claudia as well, and even characters like Ezran, who will have to wrestle with darker parts of his emotions/personality next season as well with Runaan. They've all got light and dark inside them, and learning how to walk in that balance and still break away from the Cycle / Aaravos is useful. As Ezran said in 4x03:

I just want to yell stop. But that’s not enough. It won’t work. I think about a positive vision, a faith we can all share, that we might build a future together in hope. A future where we can be safe with each other.  But… It’s not that easy or simple. Because people are still hurting and they are still angry. We can’t ignore that, or pretend it will go away. Somehow, we have to hold it all in our hearts at the same time. We have to acknowledge the weight of the pain and loss, but open up our eyes and allow ourselves to hope and maybe forgive and love again. We have to give today’s children a chance to inherit a future filled with peace. To give them that, we have to hold pain and love in our hearts at the same time.

Claudia's love for her family led her to ruin, but it can also save her through Soren. Callum's love for Rayla led him to ruin, but it has also saved him (and again, we'll likely repeat this pattern). Ezran's love for his father will lead him to anger but also pull him out of it, just as Viren's love for his son caused him to begin his journey of terrible things, but also guided him to do one final, truly right thing by the end.

Love is light and dark. Claudia, who's been walking in shadow, needs the full light - the sun, in the form of her brother. She needs to accept and see the truth of what's happening and step fully into the light. Callum, who has been routinely worried of the dark within him, needs to the reminder that he's not all he is, that he has light of his own inside. In doing so, he can break Aaravos' control over him and give hope to any other dark mages / humans in general that no level of corruption is too late to come back from, and that there is always light amid the darkness.

Life and Death

The stakes have never been higher as Aaravos and Claudia are on the warpath, determined to destroy the Cosmic Order and invert life and death. With the world’s fate on the line, our heroes must be ready to sacrifice everything to save it.

In Bloodmoon Huntress, we get a very different peek into how the secret of the Moon arcanum can be thought of through Runaan, who is peak Moonshadow-sacrifice elf man:

Moonshadow form is only achieved when we understand the balance of life against death. Balance is weight against weight, and to understand the weight of death you must feel the weight and value of another's life. Think of those you love, of who you hold most dear. Now think of the souls who have touched your life. Understand that each time your weapon meets its target, each time we fulfil our duty, the potential for that soul to change a life—to love another—is gone. We may remove hate, but we remove the potential for love as well. Moonshadow form is only achieved when we reconcile this balance between life and death.

While Claudia with her hair and dark mage-assassin parallels could unlock the Moon arcanum—especially if her perspectives continue to change—I think Callum as the Protagonist is better placed at this time to be the one to understand the balance between life and death in a season where the antagonists are trying to invert/destroy it. Him therefore understanding appearances and changes, how to control his own and see through others', understanding that balance between life and death, feels very fitting.

Callum's fear of Aaravos and dark magic comes from the fear that he's changing—"I hope you're careful, cause [magic] can change people" / "the corruption takes innocent creatures and changed them"—and that death would be better for him than life if he goes too far. Learning these things aren't the case and that he can get back to the middle (and indeed, "real trust is about accepting even the dark parts we will never know" could be about Rayla accepting he may not entirely swear off dark magic again, the same way Viren kept that door open to do Good) would be useful.

It would also mean the two mage characters most drawn into Aaravos' darkness get the Moon and Sun—the arcanums most associated with light—to banish said darkness as well. Claudia finally being an uncorrupted light and chasing life, not death, and Callum, learning how to be balanced and that he can maintain his identity / use Aaravos' book and key without fearing that he'll lose control.

The fact that Claudia's eclipse imagery only started the same episode she lost her brother, and that her Laurelion dragon-scale necklace is very Sunfire-y looking...

Conclusion

TDP for Callum and Claudia has always felt like a parallel coming-of-age mage story between the two of them, leading to the amount of similarities and diverging plot beats the two have, down to doing the same spell at the same time but in different locations in S6. The Sun arcanum is associated with truth, light, and healing, whereas the Moon arcanum is associated with change, life + death, and secrets/love. All of these things could be nicely brought to a head with Callum and Claudia each unlocking an arcanum of Moon and Sun respectively, showcasing their differences, their continued room to grow, and light amid the darkness.

While I could see alternatives like Sun for Callum and Moon for Claudia, Stars for Callum or no arcanum for either of them, I think these are the ones that fit best at this time / Stars will likely get saved for arc 3 given that arc 3 will probably focus more on rewriting destinies and the Cosmic Order as antagonists.