New hint

The following process was pretty fast, as the leader of the intelligence department of the Elemental Sea, while Bell impersonated the leader of this branch of Black Wave, she had the information of all the assassins that were registered.

She planned to give them all away to Aurora as part of her last gift to Aurora, in other words she basically controlled the underworld of the Elemental Sea, so it was easy for her to summon all of them at once.

Not even half an hour later around one hundred people arrived at this secret underground space, unlike the other branches of Black Wave, this was the one who had the less members.

The reason behind it was simple, Bell wanted to keep a tight grip on the organization, so while on the surface she accepted new applicants, everyone who didn't meet the requirements she had, was killed by her during the "tests" that they needed to do in order to join.