Joadain was very weak and he hasn't gone through any rituals of becoming the King of the Cult. King Gholad and Princess Eliza were the only hope the Kingdom has to save them from the Vampires. The Oracle knew without King Gholad, more immortal enemies of the Cult will arise especially the Werewolves knowing that there's no great Powerful leader to protect them.
With that King Gholad must not die, unfortunately for the great Oracle, but he wanted King Gholad dead and seeing how powerful Princess Eliza is, she could be a threat to the Outside Immortals just like King Gholad. Probably making her Queen and taking it upon himself to slip the mantleship to the hands of Prince Joadain. But It wasn't King Gholad who is about to get beheaded at the moment but the Princess and it means King Gholad
