[IC] New Crobuzon & the World Beyond

[GM Marvel]

[OoC: Going with your thoughts - and I'm neither confirming or denying you're on the right track - the Stain could theoretically follow along with anyone who traveled into any plane that can be easily reached by the Prime Material.  The Ethereal Plane in particular could be your best bet for the Chaos to spread since the Ethereal closely borders almost every Prime Plane.  Once in the Ethereal it could gain access to other Prime Planes through Color Pools, through people coming and going from the Ethereal (like with the Ethereal Jaunt spell) or maybe even just peering into the the Ethereal to detect ghosts and other creatures.  From the Ethereal the Stain could then also infect the Inner Planes (elemental planes).  ]