The Trials

Beelzebub was already a Peak Divinity Realm expert when he was sealed. However, that special time when one can try to break through into the Celestial Realm would never arrive for him. Soul Devils' cultivations were different from all other races of the Universe, after all. 

The Soul Devils always thought that there was only one way to get that chance, and that was to acquire Semi-Celestial Realm or Celestial Realm souls to absorb. Beelzebub always thought he would need to create some plan to kill a Semi-Celestial and that it would be extremely risky. He never expected that two of such souls would one day appear near him.

In fact, there weren't only two, but six of those souls. Let's not forget that two Semi-Celestials died in the battle at the Foundation Pillar, while another four were killed by Hyeoumu. Still, even with Beelzebub's senses, he was only able to notice these two souls.