The swarm of dragons that came numbered in the thousands. They all had obsidian scales like Azzarilth. The worst thing was, these dragons didn't look like they were newly born offsprings. Their appearance was that of adult dragons.
The reason for this was that this was not a brood produced after Azzarilth was freed. Even with the Eldritch Beholder's secret technique, only her kind could grow that fast. Dragons took an extremely long time to mature aside from their abysmally low birth rate.
The Eldritch Beholder's secret technique helped Azzarilth solve her birth rate problem but she couldn't do anything about their slow maturing process.
This incoming swarm was the brood that the Eldritch Beholder helped Azzarilth make during the ancient war. At that time, Azzarilth's brood had just been laid. She hid them away because they couldn't hatch yet. Hence, this brood didn't join the ancient war.