The half dead body?

As Cassian stared at the bloody scene in front of him, shocked and for some reason annoyed, he began to take in the details. The half-dead body wore clothing similar to his own, leading him to deduce it was likely a servant. The shoes were the same as his, and he guessed it was a man, given the pants, though he had no intention of confirming that by looking any closer.

The cause of death seemed clear—his upper body had been blasted apart, some of it evaporated, with chunks scattered across the room. The remnants had splattered everywhere, including on Lady Katherine's face and hair, which Lucy and Fuyuko were hurriedly cleaning with worried expressions.

Cassian guessed that Lady Katherine was the one who had killed the person, but he wondered why and how. The brutality of the scene suggested the use of magic, which made sense since Lady Katherine was a mage. He could only speculate about what spell she might have used and why.