All this time playing Skyrim, and I still have no idea what 'psijiic' is.

It's an order of powerful mages, whom remind me of Jedi Order due to their forecast and things like that. You get to meet them in one of the side quests though.

The order of the PSJJJ is the original order that studies magic in Tamriel.
Both Mannimarco (who becamse the necromancer god) and Vanus Galerion (Who founded the now defunct mages guild)
are people who left the Psijics in order to pursue their own endeavors without the restrictions the order layed upon them.

In Skyrim we learn that they do not agree with the Thalmor politics that grip the rest of the Altmeri islands and have obscured their island from the world again, meaning that no-one can get to it they do not let through.