From Arc Quotes

“Once Incarceron became a dragon, and a Prisoner crawled into his lair. They made a wager. They would ask each other riddles, and the one who could not answer would lose. It it was the man, he would give his life. The Prison offered a secret way of Escape. But even as the man agreed, he felt its hidden laughter.
They played for a year and a day. The lights stayed dark. The dead were not removed. Food was not provided. The Prison ignored the cries of its inmates.
Sapphique was the man. He had one riddle left. He said, "What is the Key that unlocks the heart?"
For a day Incarceron thought. For two days. For three. Then it said, "If I ever knew the answer, I have forgotten it."
--Sapphique in the Tunnels of Madness”
― Catherine Fisher, Sapphique