The Totem of Nehual is something of a Maltese Falcon. Many have spent long periods of their life searching for it, spending vast sums of money, and in the end never finding it, or only finding it long enough to lose it.
The PCs find a strange helmet, when donned it will take control of that PC and speak and act trough him/her. It explains that it is benevolent and needs their help in a quest and that it will stop possessing the PC when the task is done. Great if a player is gone for a number of sessions.