Wizardry 4 is a nice, though rather challenging, adventure game that's disguised as an RPG. The idea is that you're the evil wizard Werdna, and before the events of the story, an adventuring party rudely interrupted your research on an amulet, defeated you in battle, and stole your amulet. Years later, you've woken up from your coffin, at the bottom of a 10 floor dungeon filled with do-gooders who are out to get you (along with the ghost of your rival Trebor, who will *kill* you if he catches up to you), and YOU WANT YOUR AMULET BACK! So, with the help of monsters that you summon from pentagrams, your task is to escape your prison and retrieve the amulet.
Combat plays like in other classic Wizardry games, except that you're on the monster side, and you only get to control yourself, not your summoned monsters. On the other hand, the game is filled with some difficult adventure-game style puzzles. Be aware, however, that the game can get quite mean at times. (If you've never had a game over as a result of an enemy casting MAKANITO, you have not played the game enough to judge it fairly.)
Fortunately, the game does offer 8(!) save slots, with the option to copy saves to other disks if that's somehow not enough. Also, unlike other classic Wizardry games, there's no permadeath style autosave system, so even if you manage to teleport yourself into solid rock, you can just reload your last save and continue from there.
Note that it is recommended to play Wizardry 1 (also, I believe, not sold anywhere) first.
The Last Remnant (2009)
This SquareEnix jrpg was delisted on Steam a couple years ago and cannot be bought anywhere any longer. It was supposedly due to a remaster in UT4 that came out for PS4 and the Switch, yet no news about a PC port. Ridiculous and kinda scummy. Really enjoyable and often overlooked jrpg.
If you're going to mention this game, it's perhaps also worth mentioning Unlimited SaGa, another game by the same developer, which didn't sell well due to its rather unusual and obtuse game mechanics. (Many players, for example, had difficulty opening up treasure chests, and the way health works is more complex than in other RPGs, as characters (and enemies!) can still continue to fight at 0 HP.)
Dungeon Master 2: The Legend of Skullkeep
Don't forget Chaos Strikes Back, a stand-alone expansion to Dungeon Master which has an incredibly complex and non-linear dungeon to explore. It's also more chaotic, with certain items being placed in ways that might as well be random (they're initially on gigglers in unreachable areas, but then they move and are teleported), and with each branch of the dungeon sending you to different starting spots with invisible silent blinking teleporters.
(Game never got a DOS release.)