This page explains what you need to do after defeating the six dragons in Chrono Cross: The Radical Dreamers Edition.
After obtaining the relics from the six dragons, there’s one more thing you need to do: get back Serge’s body. However, the game isn’t very clear about how you achieve this.
Guldove (Another World)
First, return to Guldove in Another World. It’s where you ended up after escaping Viper Manor early on in the game, a short distance west from the main island.
Upon docking, you’ll stumble upon an assault by Porre’s military. During the chaos, Orlha will end up fighting you. Don’t worry too much about hurting her; she’ll only be wounded.
After things settle down, Orlha will hand you her Sapphire Brooch. Now head for the shaman’s tower at the west side of the village. Inside, show Direa the Tear of Hate via the key items menu. She’ll hand you the Dragon Emblem.
Guldove (Home World)
With the Dragon Emblem in hand, visit Guldove in Home World. There will be a guy blocking the entrance to the shaman’s tower in this world. Show him the Dragon Emblem through the key items menu and he’ll let you in.
Inside, Steena will give you the Dragon Tear if you’ve obtained the six dragon relics. She will also join your party.
Fort Dragonia (Home World)
Once you’ve got the Dragon Tear, you should travel to Fort Dragonia in Home World. In case you’ve somehow forgotten where it is, it’s accessible via the east shore of the main island. If you haven’t gone through Mt Pyre in this world, you’ll need to do so.
In any case, when you’re ready, enter the central column in the next room, where the elevator is. But before you can reach the elevator itself, a specter from the past shows up to stop you.