You had just made your way back to the Eldhaime Keep but it appears that something awful had happened during your absence.
Eldhaime Keep was attacked because it was revealed to Lilith that Astaroth was hidden there, which ended with her making quick work of the people in the keep.
Donan is somewhere inside and you will need to find him to figure out what can be done now that Lilith has attacked.
How To Get In Ruins Quest?
You will begin In Ruins quest once you have completed the Buried Secrets quest, which ended after you spoke to Captain Antje.
In Ruins Quest Objectives
1) Enter Eldhaime Ruins
After speaking with Commander Antje, you will need to head over to the entrance of the Eldhaime Ruins and open the doors to proceed inside afterward.
2) Find a way to the Great Hall
Once you have entered the Eldhaime Ruins, you will find yourself in the Stormed Battlements and will need to make your way to the Great Hall.
The path will be pretty straightforward and you simply need to continue exploring the area until you reach the Great Hall.
3) Find Donan in the Great Hall
You will find Donan once you go down the stairs in the Great Hall as he will be sorting out the dead Knights who died during the attack.
Speak with him and a cutscene will begin where he continues to sort out the dead and the two of you talk about what has happened.
He then tells you the truth about Astaroth and later asks you to accompany him as the two of you will begin searching for Yorin.
4) Enter Eldhaime Barracks
Donan will hurry towards the entrance to the Eldhaime Barracks and once he goes through, the quest will be complete.
In Ruins Rewards
After Donan has rushed into the Eldhaime Barracks, the quest will be complete and you will receive XP and Gold that is based on the current level of your character.
You were able to find Donan but it seems he was left unharmed as Lilith may be toying with him to make things worse later on.
It turns out that Donan did not destroy Astaroth and instead, he trapped the demon within a Soulstone, which Lilith is trying to get.
Yorin is nowhere to be found, which results in the two of you beginning your search in the Eldhaime barracks to find any traces.