Complex sentence: Achelous turns out to be really melancholy and, reckoning he's putting them out of their misery, attempts to drown Piper and Jason, but they manage to cut off his horn (which turns into a cornucopia) and escape.
Split into simpler sentences: Achelous turns out to be melancholy and reckoning he is putting them out of their misery, he attempts to drown Piper and Jason. They manage to cut off his horn (which turns into a cornucopia) and escape.