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It feels like fire burning inside of her. A power she can't deny; a force she can't suppress. It's called the Phoenix, and it believes in cleansing by fire.
She can feel as her body is taken over. It hurts, it burns. It's death and destruction. It's every nightmare she's ever had since she felt her powers spark within her years before.
It strengthens her for a time, makes her more powerful than ever before. But the fire can't be contained. It can't go backward, only onward. Once it ignites, it burns bright.
That's how it gets her in the end. A blaze of fiery glory and then Jean is gone (burnt out, exiled—forever) and only the Phoenix remains in the shell of her person.
It burns beautifully as it rains chaos and devastation on those she loves.
She should be glad she doesn't have to see the suffering wrought by the Phoenix, but she isn't. How can she be when she's ceased being? A final tragic irony in the life of Jean Grey.