Since you included the Diablo Series, I’ll assume that ARPGs are in consideration. Path of Exile

Caves of Qud

Hack, or its modern incarnation, NetHack. (No relation to the .hack comment. That’s a game based on an anime. Hack is a Roguelike from the 1980s, came out shortly after Rogue itself, follows a very similar format, and is arguably if not definitively the oldest true Roguelike currently developed. I mean, you can get NetHack on iOS and Android, not to mention all modern computers, and it still looks like it’s running on UNIX in the 1970s. ASCII art and all. Though there are “tile sets” that give it graphics, the original does not have any [graphics] to speak of (aside from the icon, I suppose). You are the @ symbol (well, that’s the symbol for humans), floors are periods/full-stops, walls are dashes and pipes and plus symbols for corners, dirt paths are pound/hashtag, and most monsters are letters (and uppercase is not the same as lowercase; d is dog and D is dragon, IIRC).

It’s a procedurally generated game. You start on floor 1 of a dungeon, and you have to find stairs down (>) and advance to the 35th or 37th (I forget) floor, at which point the Amulet of Yendor has a chance to spawn. The game is won when you exit the dungeon via the first floor. The bad ending is doing it without the Amulet (including right at the start!). You live a long life in obscurity. The good ending is exiting with the Amulet, in which case you live a long life, rich and famous. If you die, you get a tombstone showing what you accomplished. And you can potentially leave a ghost behind which you can fight in later runs (and get all those items if you defeat it).

The game is based on D&D, so you can level up. It’s also based on Tolkien lore, and there are a ton of neat things you can do (for example, engraving the name Elbereth, IIRC, into a floor space means certain enemies treat that space like a wall. Walking over it gives them a chance of being able to pass over it. It’s also turn based, you pause the game by not doing anything. Time only advances when you move. Hunger is also a thing, so you have to find food, or eat what you kill (some enemies are poisonous, and some give you perks, like eating a Floating Eye sounds disgusting, but you can see life forms through walls from pretty far away.

Did I mention the game is 100% free? And actively developed?

https://www.nethack.org/common/index.html ← Latest version released February 2023. So not actively actively develped, but updated more recently than, well, most games in this thread! Considering this is a game from the 1980s, that’s pretty impressive! They’re still fixing bugs and adding features.

Pathfinder: Wrath of the Righteous takes hundreds, for some thousands of hours to complete due to the complexity and sheer scope of it.

Daggerfall is long as fuck (in universe, the adventure takes at least ten years.) The dungeons are massive serpentine mazes. Multiple guilds and factions, although they don’t feature overarching questlines - lots of radiant quests, but they never really feel boring.

It’s also fairly difficult - especially if you build a character without cheesing it with a guide. You need to be juggling multiple saves to prevent yourself being trapped in a certain death situation, mess up a quest, etc.

There’s a modern remake, Daggerfall Unity, which a lot of people say is a good way to play it nowadays. The original DOS version is quite playable through DOSBox though, and there’s lots of little quality of life tools that you can find online.

Daggerfall, maybe? Beat the game, side quests, visit every location.

I mean, if you’ve played everquest 1 I feel like you’ve already hit the lore duration jackpot. It’s also more complex than most by far if you include classes that take a lot of skill to play well. Big difference between skilled and unskilled players for sure

Not really fantasy, but the Kingdom Come: Deliverance games are downright punishing and can take a long time.

Think realistic Middle Ages sim where your character is a weak little blacksmiths helper and has no clue what they’re doing.

I’d toss .hack//IMOQ into the mix, such a grindy game and borderline impossible if you don’t over-level yourself, several allies, and your gear

It’s gotta be one of the early ones, before they figured out how to balance things. Something like Avernum, Geneforge, Wizardry, or Wasteland

You could get into Dragon Quest games, if you want old school JRPGs. Complexity might be a bit lacking given the era, but doing the old NES ones without a guide is excruciatingly difficult and you can lump on RetroAchevements to add more pain. RA generally adds additional challenges to any older game so you have to play a little min maxing to accomplish them.

I mostly thought of it since RA is beta testing multi sets with DQ9 and getting every accolade might be one of the most insane things I could ever suggest.

Wizardry IV

The 7th Saga. SNES.

Longest grind? Runescape? Phantasy Star 3? disgaea if you don’t know what you’re doing