@Psychopomf  
A bit late to be answering, but-
 
The terms Seelie and Unseelie refer to creatures from Celtic folklore. Basically, they’re just terms for good and bad faeries; the Seelie are benign, mischievous, or generally light-hearted faeiries, like púca, brownies, and knockers. Uneelie are the malicious, evil faeries that attack unprovoked or steal you away in the night or what have you. They include, but are not limited to kelpies, changelings (whose concept is not too dissimilar to changelings in the show) and boggarts.
 
It’s all really cool stuff and if you’re interested in reading a fic that incorporates the Seelie and Unseelie, look up Mendacity by Dromicosuchus on FiMfiction.

This reminds me of artwork of Mushishi.  
I imagine the parasprites and poison joke are the mushi.

^^  
http://www.youtube.com/watch?v=AeMc0cygHNw

@Psychopomf  
Those are intriguing questions.

Lovely picture. The setting is very fitting for them ^^

So if they are the Unseelie, who is the Seelie?
 
And what did they do to them?