Adrian disappeared from Alabaster Fortress as he no longer needs to be there. He knew Alabaster Fortress is done for which is why he needed to get out of there. The entity with a powerful aura that he felt is as powerful as Cristobal.
The fortress commander of Alabaster Fortress was only about level 100 when the first siege happened. Adrian speculated that the fortress commander is about level 150 or higher by now. His guess is not that far from the truth as the fortress commander is about level 170. The fortress commander might be powerful but not powerful enough to contend with the undead he saw.
The doom knight who is half a second late in killing Adrian though is not happy as it scoured the battlefield to find the demon. Due to Adrian's mischievous antics, the doom knight wasted ten more minutes to find him. The undead even took their time to demolish the Alabaster Fortress.