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Broken piece of trash quest.
If the knight aggros when she still has a bubble around her or if you're suddenly being attacked by invisible enemies, just reload.
She's completely invincible in that state since she's basically wearing the prison you're supposed to free her out of, and her allies don't actually physically exist so you cannot retaliate whatsoever.
Feel free to brain her immediately after freeing her; she's worth a smooth 40k and man is it satisfying.