The way it's portrayed in the book is unsatisfactory. Он должен быть как по Пушкину, не менее. And narratively, it should be a temptation; the temptation to remain, to rule, in peace and prosperity. So we'll leave it where it is in the story, but we'll also insert it earlier. There, the PCs will *briefly* visit it; we'll put it into a place in the story where the PCs are hard-pressed, and can't vend or buy, or restore their resources and recover. Then we'll take them to the island, and they'll see its majesty and glory; but Prince Gvidon is old and dying, and the natives fear what will happen when he goes. Only a hero of noble blood, and an outsider, can rule them, else they'll fall to squabbling and ruin. Gvidon recognizes the nobility and heroism of the PCs (if any of them indeed have demonstrated true heroic traits), and invites one of them to rule in his stead when he goes. They'll probably refuse; ok. He is saddened, but says that in any case, they are welcome guests there, so long as he lives. When the PCs visit the place later, it's under siege by the beasts of the sea; those are exactly the combat challenges and encounters they're to have. Gvidon is weak, and half-delirious; he will die soon. This is the last chance for any of the PCs to choose to stay and rule. If they don't, well, they can hold off the enemies long enough for the children and women to evacuate to elsewhere in the Thrice-Tenth Kingdom, but the men will stay, and fight, and lose, and the Island of Buyan will fall to the enemy, who is Chernomor, warlord of the ocean giants, mighty and terrible.
---
"I have lived for many lifetimes; and I have lived many lives. I have done such glorious things, and seen such wonders, that I can wish for you no better fate than that you should see the things I've seen, and do the things I've done. I have... walked the Stair of Stars, and held newborn worlds in my hands, and felt the solar wind in my hair; I have seen more creatures, strange and beautiful, than there are grains of sand on the beach. And the last thing that I wanted was to live one perfect life, as a mortal woman; to love, and be loved, by a mortal man; and to be by his side til the end of his days, and mine. This, too, I have now done. All that remains for me is rest."
She smiles at you, with a look of kindly pity. "I have already gone. This is only an echo, left behind for those who would wish to hear these words from me. And now even the echo must depart."
