It's called "Double-jeu" and you can find it on Focussed On Nick Lea Discussion Forum (Krycek-centric Slash section) or on my site at kaNd's Wonderland in HTML format.
It's M/K/Sk (yes!), absolutely NC-17, long (>19,000 words), totally PWP, with a lot of toys.
The idea? It's Alex Birthday. Fox and Walter want to make it a memorable day... and night.