What terrible faces Percy when he decides to use the pearls.

I don't completely understand your question as written, but Percy faces a huge dilemma when he learns that each of his three pearls will only protect one person...... thus, he can save himself, his friends, or his mother...... but he cannot save them all.

“Only three,” Hades said. “What a shame. You do realize each only protects a single person. Try to take your mother, then, little godling. And which of your friends will you leave behind to spend eternity with me? Go on. Choose. Or give me the backpack and accept my terms.”

I turned and faced my mother. I desperately wanted to sacrifice myself and use the last pearl on her, but I knew what she would say. She would never allow it. I had to get the bolt back to Olympus and tell Zeus the truth. I had to stop the war. She would never forgive me if I saved her instead. I thought about the prophecy made at Half-Blood Hill, what seemed like a million years ago. You will fail to save what matters most in the end.