PCs are told to act as Escorts to an important artifact (or set of artifacts) which will be transported from one point in coastal Europe to the East Coast of the US. It will be transported by steamship.
Miskatonic University hires the PCs.
Events will occur on the ship that will cause the PCs to believe something sinister is going on involving the artifact(s). People will begin to disappear and ghost sightings will be reported elsewhere. Blood trails end abruptly in some disappearance cases. Others hear screams in the middle of the night of those that disappeared.
At one point, the vessel will take a sudden course change northward and the bridge seems to hauntingly be taken over.
These artifacts have been hidden for centuries in the ruins of some ancient temple under a binding spell. Unfortunately, the spell was broken when the less-than-learned archeologists dug them out. Now, the artifact(s) have summoned their “Keepers” who exists in a reality just outside our own. They are virtually invisible to the PCs (unless someone is holding the artifact(s)).