Youel didn't hide anything from Elena after making her decision. According to her estimation, that Cataclysm class monster was at least over level 3000+. As for why, she was sure about that, it was because she could appraise anyone below level 3000 including the Apocalyptic Monsters. So, as she could not appraise this monster, it was definitely over level 3000.
When she saw Elena for the very first time she got overjoyed for a moment as she was unable to appraise her, but soon she remembered that World's Wills are unable to appraise any World Savior and she got depressed again.
Other than the monsters level she was not sure about anything more as she could not use appraisal on it. But she told Elena about the three disaster rank subordinates it had. She was able to appraise them and they were also quite strong.
