Illya was attacked by a villain who turns girls to stone and sells them. She bravely tried to fight back, but was petrified instantly without being able to do anything.

Chloe was elsewhere at the time, but unexpectedly the magical link between her and Illya transferred the petrification spell directly to her. She suddenly and quickly turned to stone out of nowhere, with no idea what was going on.
The villain collected them both as hostage statues.

When Miyu came to rescue them, the villain forced her into a twisted game. He'd petrify her slowly and painfully from the bottom up, and if she was able to maintain a humiliating happy smile and double peace sign until the end, he'd turn everyone back and let them go. With no other options, Miyu agreed.
She almost made it. Miyu endured it until near the very end, but at the very last second her smile faltered, losing the game by the skin of her teeth. As her mouth froze in a grimace of pain her eyes filled with despair as she realized her failure, and shortly thereafter petrified as such.