>>713682439If Inquisitor Lavellan romanced him in DAI and Rook tells her she should stay with him, she (together with the fragment of Mythal locked in Morrigan) will convince him to stop. There's a cute ending slide where he and the Inquisitor hugs and Varric's voice says he's finally reunited with his true love, and it's implied the Black City will turn into a paradise/will be healed thanks to their love, since the Fade reflects the emotions of those within it.
If the Inquisitor didn't romance him, you can either convince him by still using the fragment of Mythal, and he will roam the Fade trying to heal the Blight from there, alone but redeemed. If instead you hate his ass, you can lock him away in the Fade prison forever or lock yourself with him (if your companions are all dead). Honestly the Solavellan ending is the only good part of the game, they even used the Lost Elf musical motif from DAI and you can hear the sheer difference in quality between Trevor's 8/10 OST and Zimmer 0/10 slop.