A black hole. This meant time dilation field against time dilation field. Luckily for Atrus, it had more than just its own defenses to protect it. The whole Galaxy was like a mafia, in this case, a mafia of black holes and stars that were close to it. Each black hole representing one member of that mafia like a family. If you attacked one? The rest of the Galaxy would defend their 'family member.'
In this case, the black hole had been summoned. This meant it was done by magic. To the pulsar that swept across Vaccus reflecting off Nil and its anti-magic substance, this was a big no. Magic could be done in Vaccus, but if the pulsar hit it before it was cast, you were done for. In this case, the black hole summoning was too late.
The spell was nulled.