Ultima Underworld is actualy mother of ARPG and father of FPS games. By beta version of that game Carmak was inspired to make his simplified raycasting technology that evolved in wolfenstein 3d. Most technologically advanced game of it's time it may be not directly follow lore of ultima (in that time every ultima game had inconsistes basicly... actually every part of ultima had more of them than elder scrolls games)
but it is truley follow meaning of ultima - not only by mantras, but with your relationship with actual world and understanding eveyone and their life, understanding complex ecosystem of world.
Like how important dialogues and relationships in that world - every person have it's conditions from very friendaly to upset and angry, often even hostle monsters may be not that hostel if you not do aggresive things about them - not steal their food or attack. How much days you spend in abyss, in what condition your food (fresh or not much), how you resolved puzzles - in many ways more important than things like combat.
On every floor of abyss there communities - different tribes of humans, goblins, dwarves, lizzards and even ghouls and trolls that in complex create big living community that you need to interact with. Dialogue system there what is become "classical" ones but in this example it is in many ways showed how it should be done best. I often say that part where you should learn lizzard language in realtime with help of pisoner that silent like fish and showing meaning of lizzard words by sharades and you need write them on paper is absoluetly awesome and game full of moments like that.
Leveling by praying mantras, magic system by collecting runestones, deep inventory system whre you need stuck everything in actual bags and wallets and you need actually manage your inventory, not "you have X kg in your inventory", trading is actual barther where you like "I give you that, you give me that.. offer?". And even there gold parts andcoins, they used not as primal currency most times.
Detailed enivrment for it's time, innovative dynamic music and basic, but very good plot that you remember. And second game in much more ways connected with main ultima plots but this is another story. Truley, this is one of the best Action RPG (in cassic meaning of this word, not skyrim or wedzemin 3)
However, like it's sucsessors System Shock 1-2, this game may have not very intuitive for modern people controls - as many games from it's times (like wasd become a true thing only from Quake), as it game have THAT control system of turn based first person tiel based blobbers, but IN REAL TIME. You know that system if you played TES Arena (that by itself was in many wasys based on Ultima Underworld and basicly an Elder Scrolls Legen Battlespire was Underworld clone) or it was opotional control scheme in tes2 daggerfall even. Graphics - is example of one of the most early proper fake 3d with 2,5render, in many ways more advanced that wolfenstein and even sometimes doom, but with resolution and software render that far beyound what you accepted as normal today.
But you know what I may recommend for you? Not in any way as inssult by any kind, but actual honest recomendation based on personal experience. Try to go ddep BEYOND YOUR ZONE OF COMFORT. You know that I most probably kid compare to you? I start playing games from 2006 or something, get my personal PC in end of 2008, games of mine chldhood was GTA Vice City and San Andreas, then Call of Duty 5 and Stalker, Counter Strike Source and Condition Zero, Warcraft 3. Games that by any defenition is far from "old" or "assburger". Many control schemes, genres, old graphics was for me ugly and unplayable, but I go deeper and deepre, tried to understand many old games, understand old graphics and classic gameplay - and for many ways I rewarded by discovering so many things, a truley masterpices that created me as person. From Morrowind, that at first try for me was ugly and clumsy to wizardry 6 that at first was absoluetly unplayable. Now I feel there like in home, I prefer a lot of style of clssic game art and grpahics, I feel myself comfy with old control schemes, like daggerfall where you need to move mouse to hit someone or jurassic park trespasser where camera and hand movment separate.
And more you go deep inside it - more less you want usual and casual games. It is like after you seen A GOD, anything else look more gray and boring. I shurely don't want go back to skyrim after morrowind, and I'am shure I will not spend many times in call of dooty or battlefield.
If you want disscus classic grphics and design, maybe engines and technologies, in more depth, I will shure be happy, this is my favorite topic.
About Thief 1-2 games - yep it was made by same guys as Underworld and System Shock, but yeah, it much more modern in therms of controls, don't worry. It is not RPG, but not and FPS too. This is most awesome first perso stealth simulator was ever created, with nice style, lore, atmosphere. With very complex gameplay around actual shadows, noise. Basicly on every level you have goal - to steal something or do something. Level is complex labytinth that you have on map only in style of actul picture your hero did himself on piece of paper. You need use covers, stay in shadows from patrooling guards and all other kinds of creatures. Using your arrows with water and mood, be able to use your rope everywhere making for free of movment, also there a lot secondary goals from stealing aeverything to not kill anybody and with increacing difficulty it all become more difficult, where you need to think tactically. I remember first time playig how I maded one guard unconscious and moved body in a puddle... and after time he drowned and died and I lot game lol.
Classic thiefs and 2014 YOBA remake-seqel is actually main topic of that famous video Thief vs. AAA gaming:https://www.youtube.com/watch?v=jPqwDGXxLhU
Well, if it ishard to start, nothing bad to start from videos on youtube, about learning facts, technology, secrets and community of all games you want get into. maybe even watching quality let's play may be not a bad idea. All this games come from time where you need read book andmanual before playing it and have actual threshold of occurrence, but belive me, it's worth it.Stalker... well, at first it much better play on russian :D