The God's tomb.
Grey told his father and the others about the news of the Gnomes coming into the God's tomb, sending every top Sovereign from their world.
They've gone to meet up with his mother, so the group was complete. While going to where his mother was located, they managed to kill a good number of Necromancers.
After teaming up with his mother, they've killed a good majority of the Necromancers present and all the vessels he knew about have been killed. He knew there were a few he didn't know, but among the Necromancers they've killed, he was certain a few of them were vessels. His father didn't bother if they were vessels or not, as long as Grey confirmed they were Necromancers, he killed them on the spot.