Ymir had used so much mana to summon so many spectral copies that he couldn't feel much difference when one of them disappeared. However, the real problem there was that he couldn't make it disappear, wherever it might be, and Femradiel noticed that.
Another thing that Femradiel noticed was the fact that Ymir couldn't make Void World disappear… maybe it was because it was the spell that doesn't alter reality like other spells. It alters by making the target stay unconscious. Still, since the target this time was a concentration of energy, nothing was left behind.
"Did you see that Hypnos?" Femradiel asked via Telepathy.
"Yes… but if you let the spectral copy run free, it will eventually destroy your Void World," Hypnos replied.
"Ah… I won't let it," Femradiel smiled. "This one here will have other uses."