Luke/Kronos - Curse of Achilles

Then again, Luke had been here, and he hadn’t turned back.
That’s why Luke had been able to host the spirit of Kronos without his body disintegrating. This is how he’d prepared himself, and why he seemed impossible to kill. He had bathed in the River Styx and taken on the powers of the greatest mortal hero, Achilles. He was invincible.
“I have to,” I said. “Otherwise I don’t stand a chance.”

***

The Last Olympian, Chapter 8