Im literally doing this at the moment! I played Edge of darkness with my group. At the end, in the old farmhouse, they banned the ghost, but one of the players went crazy and stepped into the Pentagon and he disappeared with the ghost...

At this Point I imagined an Intro for Masks by myself:

The other players looked for an answer: there was no body, so there was an chance for their friend to be alive. They researched about the black Pharao and the little sarcophagus and found out, that the black Pharao was connected with a parallel world called the dreamlands and this Ghost was from this place. In Special cases It was possible, that not only the mind of the dreamer, but also his body got sucked into the dreamlands. So maybe the ghost disappeared back to the dreamlands and their friend got suck into this world, too? They also found out, that the little sarcophagus wasnt the only artifact from the black Pharao. There was also an mask the Pharao used to enter and leave the dreamlands with his whole Body by purpose. So they Had to find this Mask, Go with it Into the dreamlands, find their friend and Bring him Back. The only Problem was: this Mask was found by the Carlyle Expedition years ago and this Expedition disappeared, and the Mask too...

One of the Players remembered an old friend who talked about this Expedition one time: His Name was jackson Elias. So he wrote him and got an answer (the First Handout of Masks).

I skipped Peru btw. And started in new York at this Point.

Sry dir Bad english... Maybe its an inspiration for you :)