The idea is simple - someone (or possibly several someones) in the fandom of your choice have been replaced by Skrull infiltrators, or possibly has always been a Skrull infiltrator. The challenge is to show what happens as a result...
For more on the Skrulls and the invasion see
Any genre you like, any pairing or none, my own preference is for short and funny but that's just me, write what you like. I'm not up to organising this as a ficathon or anything of that sort, but if anyone wants to run with this idea please post a link to your story as a comment below.