It was Derrick that rushed in, squeezing himself between Ariel and the magic beast.
Derrick died instantly due to the blow by the magic beast.
Apparently, the transported Sylphy managed to subdue the magic beast, but... that's a different story.
"If Derrick didn't stop his incantation, he might have subdued the magic beast, and wouldn't have died."
"But if so, Ariel-sama might have died instead."
"Indeed. Death, or at least a serious injury."
Sounds like Derrick bet his life to protect Ariel.