Ars Magica: game mechanics wise it’s simulation with lots of cool scenery to chew. Plot wise it can go either into mystery/puzzle solving which requires heavy lifting from the GM (preparation and being a fountain of information) or PvP with strong player characters pursuing agendas at odds with each other. Some PvP seeps in already from character building but there is little mechanical support for exploring it in the game. So definitely a game that requires heavy GM lifting.

I did a one shot GM less game for a condensed PvP experience a few years ago: The death of Rapacus. Not officially AM, no setting knowledge required but fans will quickly recognise the universe.