This is just my imagination of what alfia said to ais in the astraea record.

Maybe someday or eventually ais will be sacrificed to dungeon for calm it for few decades, each one Hundred, or each one Thousand year. Before ouranos can found another way.

The sacrifice one or in this case are ais is must be a spirit or the descendant of the spirit. As well the one which should be sacrifice must not be a normal spirit but a great spirit or the descendant of great spirit.

There's already many people similar to ais which sacrificed in the past. Between ten to few Hundred spirit to the dungeon.

Ouranos and the God's must give it to dungeon for not let something in there ascending to the surface.

This is are one of Possibility way or methode for defeating the black dragon which interrupted by someone or someone interfere in the middle process.

This is are one of truth about the corrupted spirit origin too. They are the spirit soul which sacrificed and couldn't rest in peace.