good question - I'm still learning 4E, but I'm sure if this was 3E/3.5, there would be some sort of template you could apply to creatures to give them a "far realms" feel
Also, liked that you gave a monster manual monster a new name - keeps the players guessing.
In this case, it was serendipity. I had already devised the basic story of the Suelwyn and the idea of them. Then I cast my eye over the compendium to see what I could use for some monsters to be part of them...and found the foulspawn were a perfect fit.
Without going into too much detail, the Suelwyn were the first attempt at life that the Divine beings of my world attempted to create. They were...not well made. They managed to create a great empire, nonetheless, mastering arts, culture and magic. But then the divine broke into two groups. One group created the Aewyn (the Eladrin) who rapidly appeared all over the world. The Suelwyn grew bitter and jealous, as they realized that everything they'd accomplished was (in the eyes of their creators) for naught. They decided to leave the world forever and traveled to the Far Realms. By the time the other group of the divine created all the other mortal races, it had been millenia since anyone had seen the Suelwyn.
Ironically, the rise of the younger races caused the Aewyn to leave for the Feywild, but more to be closer to their masters. But about 100 years ago, a massive war raged across the world and the bad guys, under the collective banner of the Forces of Dread, summoned horrors from the Far Realms when they started losing the war. Things looked dire for the alliance, until the Aewyn returned to aid the younger races and eventually turn Dread back.
Now it's 100 years later and the Suelwyn are coming back. And they're PISSED. The Aewyn, for their part, have been too polite to explain that many ruins and structures that the younger races attributed to them in their absence are not actually made by them, but by the Suelwyn.
...
Which leads me to a thought. It's commonly believed that the Aewyn built Castle Whyte. Now I wonder if it's a Suelwyn structure. Hmmmm. If anyone would know how to damage their structure, it'd be them.
To the Compendium!