Siege Perilous

This weeks Taming the Muse prompt is siege. Through a friend I know enough about Grail lore to search on siege perilous which leads me to a Marvel comic webpage, where I read this: "This... thing, Electro, is the Siege Perilous. A mystical key allowing us to enter world parallel to our own." So we've got a key as a gateway to alternate dimensions. The image doesn't have green light but turquoise light, that's rather close.

On the same page there's listed a group of bad guys called Reavers.

It's almost enough to make me suspect Joss Whedon is into comics.  ;-)