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Yes, the "Cloud of Despair" it makes everything around it feel bad i think...
The idea I would like to try would be opposite to that. It would make you feel better by taking your bad feelings away from you. Not permanently of course but...

For instance, lets say some one like Cordelia was feeling angry or unhappy for whatever reason, but was happy just minutes ago.
The effect of the fel essence im considering would make my eventual raevan able to "eat" that moment of unhappiness and thus take it away, making Cordelia no longer unhappy.

But as a recoil of this, my eventual raevan (if allowed this essence) would get the full effect of the emotion eaten. Meaning they would become angry or unhappy, but the effect could last far longer on them then it would have originally.

I'm just not sure how I could incorporate this idea into a working fel essence... or what to call it.

I might be able to... man this is hard...

If it helps, think of things that exist naturally. The more common it is, the easier it is to work with.

You could try something like Reverse Gravity.
Gravity is the invisible force that keeps things from floating around. Well, what this fel essence did the reverse? It keeps things from being grounded!

But, I'm not sure how you would depict that in a vial or jar, and I know Kyou isn't very partial to empty jars/vials. They don't give her a lot to work with in terms of color.

Not sure how exactly helpful this might be, but going back to your anguish idea... What about something like "Heart of Stone"? She wouldn't be able to feel any emotions of her own and would feed off the emotions of those around her.

It might be hard to pull off a completely emotionless character though...