They’re both sad in their own ways

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So, I read that he turns into her dog, regressed at least. Instead of regressing into his normal self(?) and he will be Thalia’s dog. So I’m curious how that will play out

Varkas didn't regress, he morphed his soul into Thalia's wolf body and his name as a wolf is Khan. I'm not entirely sure how he pulled this off, though. The novel just says he was groveling, crying, and praying and when he opened his eyes again after all that misery, it had already happened.

Since the original Varkas is still alive, there are actually two of them now in the same timeline.

The OG Varkas inside Khan's body (whom some readers call "VarKhan") and Varkas. They've met a few times and the regular Varkas hasn't changed at all. He's still clueless and miserable.
On the other hand, VarKhan proactively tries to help Thalia, though he can't do much since he's stuck in a wolf's body. Despite this twist, the trajectory of the main story remains the same.

Like I mentioned in my previous comment, the author likely introduced this VarKhan plotline so we could get a clearer look at Varkas's POV.

Right now I'm at Chapter 290-292, where VarKhan kills Marisen. Remember Gareth? That scum.
He asked Marisen to poison Thalia so there wouldn't be any descendants of Thalia and Varkas. After this was revealed, Thalia actually asked Marisen to leave and never come back. She didn't punish her because Marisen only did it to protect her own daughter from Gareth.

Well, VarKhan mauled her to death anyway. Getting revenge for Thalia and their lost child.
This is what I meant when I said the overall trajectory of the story hasn't changed. Even though VarKhan was able to avenge Thalia to some extent, the damage was already done. It isn't just about their dead child, but also all the other tragic events that made their lives so miserable in the first place.
Novel readers are still waiting the moment VarKhan back to his original body and break the curse somehow but until now, no progress.