Man, 'Devil Fire' was a trip. That ending had me tossing the book across the room at two in the morning, but in a good way? I guess? So the hero, Kael, basically pulls this massive Hail Mary where he lets the titular Devil Fire consume him to become a living conduit for it, sacrificing himself to seal the big bad back into its primordial prison. But it's not a clean death. The epilogue shows a single, charred hand pushing up from the ashes a century later, and it's implied it's him, reborn but fundamentally changed, maybe more elemental spirit than man. His love interest, the arcanist Lyra, is now the head of the order and has spent her life studying the seal, waiting for a sign. It's bittersweet as hell—he saved the world but lost his humanity and the life they planned.
Honestly, I found the whole 'hero becomes the thing he fought' trope a bit predictable, but the execution with the visceral body-horror descriptions of the fire rewriting his cells was pretty gnarly. Makes you wonder if a 'happy ending' was ever in the cards for someone who had to wield that kind of power.