The campaign will use Lamentations of the Flame Princess: Weird Fantasy Role-Playing. To a first approximation, it’s a clone of Basic/Expert D&D, but the author made some significant system changes:
THAC0 is gone, replaced by a Basic Attack Bonus like 3.x. (Saving Throws still work the same.)
Classes progress only in their particular niche: attack bonuses (Fighter), spells (Magic User, Cleric, and Elf), skill points (Specialist), ridiculous saving throws (Halfling) or mad hit points (Dwarf). Every character improves saving throws and hit points.
Only Fighters improve their BAB with experience. The Referee book suggests keeping armor classes and hit points within reach of non-fighters, but Referees don’t have to send players creatures they can kill.
A “Specialist” class replaces the Thief. All characters can Find Traps, Search, Stealth, Sneak Attack, etc., as a 1 in 6 chance, usually. (Demi-humans have higher chances in a few narrow skills.) The Specialist gets skill points to increase these chances, up to a 35/36 chance or x6 damage for a sneak attack.
Law and Chaos don’t mean good vs evil, but faith that life has a reason and purpose vs. belief in a random and pitiless universe.
Clerics have no Resurrection spells. Death is no revolving door.
In Lot FP, characters hunt for treasure in tombs and forgotten ruins; treasure nets more XP than killing things because things more easily kill you. The author’s published modules contain far more traps than creatures, occasionally sudden-death traps; some things can’t be fought, only out-smarted or fled.
Through rules and various descriptions, the game creates an atmosphere of darkness and dread. Player characters aren’t heroes guaranteed to survive to the end of the story, which might be an adjustment for some players.