Cael had to get very close to the Starspawns to be sure that his tactic would work. The Starspawns were quick and were protected by powerful shields. There was only one type of attack that these creatures were really vulnerable to, and Cael could only use that type of attacks in melee range.
Antimagic.
It wasn't an element like water or fire, or even a substance like blood. Antimagic was more like transmuting, or divining, in that it was a set of principles a spell could use to achieve something.
In the case of antimagic spells, it was unraveling other spells and turning them to raw mana. Something onyx dragon scales did naturally, and Cael had to use magic for.
Same magic he once wielded in a dagger against Shattered Fourteen. But now he was much better at spellcasting, and could use the same principles without an artifact.