Edge island

I have a question: I killed Crag Hack, but I can't get into the last Conflux castle! The problem is that I can't summon a ship with the light-blue keyholder – it's too far away. And with the boots of levitation, I can't fly over the keyholder's gate and the sea at the same time. The Grail is forbidden in the Conflux. Do I really have to spend a year trying to conquer this scroll? Maybe it's a bug or an oversight. But how did others manage it? I'm playing SOD.
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A couple of months later – I took the scroll, gave it to the Berserker and Vampire Lords, captured the Conflux, placed the Grail – still no victory. I brought Catherine there, nothing happened. The quest says to improve Cornerstone, but there's no such city, and the Conflux is already built to the maximum.