There were a few things Dilan wanted to know about the Werewolf King, the Witch Queen, and the Vampire King, and both the Blood Demon and the summoned Demon were basically his best sources of information in that regard.
He stopped using mana to control Kelkas because it was simply not necessary after the summoned Demon had seen what Dilan was capable of.
As for Zirach, he was not healing properly and the only thing he was capable of doing was destroying his pain sensors. This was his only way of ensuring that he wouldn't feel any pain for the next few minutes until Dilan would end his life.
"So basically, more human or human-like races like the humans on Milarn have been in existence, and it has been a very long time since the Primordial Ascension occurred in your realm.