Am I being thick or what? No dialogue options showing for incantation to finish the fight.
Try hopping worlds. This version of Demon Slayer is before instances so if someone else interacted with Delrith before you, things can get weird.
This behavior will be fixed in the next update: fix(content): Delrith interactions by Gregf36665 · Pull Request #1262 · 2004Scape/Server · GitHub