Spoilers ahead for Create's role in the fight, the boss's recovery and the unlocks after the fourth district.
The Castle of Legend opens after your victory in the Forest. It's the next part of Create's story, and your choice before the final battle changes who takes part. Your aim is to reach and defeat the Rift Guardian.
Finish the conversation first
You can continue alone as the standard hero, fight alongside Create or take control of him. If you go together, Create fights as an ally. If you choose to play as him, you control a different character.
Make your choice and reach the final section to trigger the Guardian encounter. If you're at the end and nothing happens, check that you've finished the conversation and chosen a role. Clearing the ordinary enemies won't meet this condition on its own.
Zombies and skeletons appear along the route. Use those fights to get used to moving around the castle. The final opponent has a different set of attacks. If you've taken control of Create, check your abilities before entering the arena.
Reading the Guardian's attacks
The Guardian uses a sweeping strike, a flying pass, a rock, a laser and an ice attack. Dodging in the same direction each time won't work against all of them. Look at the warning area for the current attack and leave space to move again.
If you're fighting as a pair, the Guardian can target either you or Create. Your ally can draw its attention, but that can change. Watch the windup even when the boss appears to be facing someone else.
Healing pickups can appear in the arena when you're playing the standard hero. They don't appear when you control Create. Plan how to recover according to the character you chose.
Why the boss recovers health
The Guardian has a one-time recovery. At a health threshold, it briefly becomes invulnerable and then regains some HP. Reposition during that window and resume attacking afterwards. This recovery is part of the encounter.
Victory awards 50 crystals and opens Stone Passage. You also unlock the Arena and the option to replay the first three districts as Create. The next district follows its own character rules, so the fighter chosen here doesn't automatically carry through the rest of the campaign.