8.13.2 Come under me.
The core ability of a Curse user was a sacrifice, an exchange of services, will, and souls. They can give part of themselves to attain greater strength, and this sacrificial exchange was never limited to Curse users alone. There were Mystics capable of similar feats and even Knights, who could burn through their blood to achieve similar results.
But no matter who it was, no one can compare it to the mastery of sacrifice regarding curse users.
Curse of Isolation is one of the most dangerous and potent curses in existence, and few are aware of its existence due to its strength and functions. So the knowledge about it had been lost in history, and only beings like Jester are aware of its existence.
Even such a potent curse is feeble in front of a powerful Radiant "beast" like Remira.
But that's where the Sacrifice played the part.