How do I unlock more skills in an essence?

[Frubidore]

When I scroll through my long list of essense, most have 3 out of 5 skills available, the other 2 grayed out. But a few have 4 out of 5. How does any of that work? I have demons that have obtained all their skills through levelling, but still, some skills from their essence are greyed out...

[Togles]

killing abscesses, iirc every new region has one that gives one more slot.

[Arisato]

Learn the miracle called Art of Essences 1-3. It allows you to unlock more skills your demons can learn from essences. Where to find it? Find and kill Abscess. Which 1? I don't remember exactly, just kill whatever you see.