My PCs are already in Xin-Shalst and are collecting Sihedron rings. They need just one more. They've figured out that the rune giants have them, and they are planning to sneak invisible into Shalaria during the night (as I understand, that's where the rune giants that are in charge of controling the Lower Xin-Shalst stay), find a rune giant using locate creature, and assasinate him in his sleep, take his ring, then escape.
This sems like a plan that could work, and rune giants don't really have an effective protection against that. Besides, I'd like them to succeed (we've spent a lot of time in Lower Xin-Shalast and I'd be happy to get to the Spires), but I don't want it too be too easy. I have no experience with infiltration missions and I'm seeking suggestions how to make this into something more interesting than just everything going as they planned it.
I'd also like to flesh out Shalaria in more detials, in case if it cames up. For example, how many cloud and storm giant still inhabit it? What are their living conditions? What would be some interesting places in Shahlaria that the PCs can stumble into?