Sacrum

Sacrum is a sea serpent who ended up being fused with two phantasms in a desperate move by their parents. Their parents fear for their own lives and want to either appease the local phantasms or find a way to fight them. Sacrum was unfortunately chosen to undergo an ancient, taboo ritual that ending in the fusing. From there they went on an uncontrolled rampage and killed their parents and all the others in the small village they called home.

They traveled after that, desperate to learn to control the phantasms they were now fused with. That journey eventually led them to the order for help. Whether that be help to control it or... well a more final option. Thankfully the order was able to offer help, but they were forced to join so they could be monitored.