Pathfinder: Wrath of the Righteous
Introduction
Pathfinder: Wrath of the Righteous, or WotR in short, is arguably one of the best RPGs in recent time for hardcore-oriented players. It natively comes with a vast range of difficulties, the highest of which (Unfair) requires, by design, vast amounts of game knowledge to take on.
It also provides optional "hardcore" types of playthroughs : Last Azlanti is a no-reload optional toggle, and Sadistic Game Design achievement brings in a very good baseline for what can be considered a challenge run.
Essentially, it sticks very close to the standards set by Baldur's Gate way back in 1998-2000 of having both a very low entrance fee for casual players and a very high skill ceiling for hardcore gamers.