Thief: Dark Project, released in 1997, is a masterpiece of first-person sneaking. While its graphics are two decades old by now and some of its mechanics are somewhat clunky, Thief's stealth mechanics are second to none. Very few computer games released in the last two decades even come close to the wonder of such mechanics. There are some great stealth games released after Thief, but usually they fall into the temptation of playing an overpowered assassin rather than a proper thief. Even Dishonored (2012) which took much inspiration from Thief succumbed to the lure of supernatural assassin powers such as teleportation, and great combat abilities. Such games are wonderfully enjoyable, but they do not have the hardcore stealth gameplay of good old Thief.
Thief is what it says on the tin. You play a thief in a fantasy/steampunk world. You are NOT a warrior. Neither are you a mage. You are a master thief - probably mid-level in ACKS terms - and highly skilled in stealth. However, in face-to-face melee, even a humble guard is a major threat to you. Dash into a room with your sword flailing, you will get killed. Instead, you have to pay attention to darkness and to floors which make less noise - and sneak. When in full darkness, you are practically invisible unless a guard stumbles upon you directly. Step on a noisy floor such as a metal ramp, you will make noise, and guards will come to investigate. Guards who stumble upon dead bodies or blood become suspicious and start searching for the preparator.
When using stealth properly, however, you have a massive edge over any guard. Sneak behind them undetected and you can knock them out cold with a single blow to their head or back with your trusty blackjack. Sneak well, and you can pick-pocket them without them noticing. However, you can complete most levels without killing anyone - while getting away with all the loot! This is a matter of skill. The better skilled you become in this game, the less you will find need to kill.
If you want to feel what is it like to play a mid-level thief in ACKS, this is your go-to game.
It also has a wonderful range of inspirations for your ACKS games. Almost all levels are large and non-linear. Exploration is key to finding loot (which you use to purchase alchemical tools for your next level, and also serves as your "score"). There are several *WONDERFUL* dungeon levels - expansive catacombs, a vast Egyptian-style lost city, a haunted quarter of the city, and a massive haunted cathedral/monastery.
Thief also details Law and Chaos as seen in ACKS. Chaos - the Woodsie Lord and his followers - wants to destroy civilization and return to wild bestiality. It is insidious and treacherous. Law - the Hammerites - will do what it takes to preserve civilization, including torture and incarcerating sinners in a horrid prison. They also fall into excesses of over-zeal in fighting crime, immorality, and chaos. However, Lawful they remain - protecting civilization from its enemies, hammer in hand.
Undead are frightening. Even the simplest zombie is hard to kill. You can hit it with your sword or your arrows and it will fall to the ground. However, it will soon rise again. To permanently slay a zombie, you need to use special tools - fire arrows, landmines, holy water, or flash bombs. These are limited in quantity. Avoiding zombies is typically easier than killing them. There are worse undead - teh Haunts. These skeletal warriors are murderously efficient swordsmen; in face-to-face combat they will kill you very quickly. However, a sneak-attack to teh back of an unaware Haunt will slay it.
Finally, you see the fruits of alchemy. You are a thief, not a mage, so you can access magic only through alchemical concoctions and elemental arrows. You have healing potions and water-breathing potions, holy water used against undead, and speed potions improving your jumping and running abilities. The interesting tools are elemental arrows - fire arrows (explosive, noisy), air/gas arrows (knock out enemies quietly, area-effect), earth/moss arrows (silence noisy floors), and water arrows (can put out torches and erase blood stains). You can purchase them, but here comes the intriguing part - they occur naturally! You can sometimes find moss arrows on rotten wooden beams in dark places, for example, or fire arrows in furnaces.
As in OSR tabletop games, however, you have strictly limited resources and you must manage them. You only have so many arrows of each kind and a small number of potions. You have to think carefully which tool to use in a given situation, if at all.
All in all, highly recommended. Ignore the old graphics - the game is still a masterpiece.
Its sequels, however, have a different tone. Thief 2 has an upgraded version of the same engine, but leans far further into proper steampunk sci-fi than the original game; it also has less fantasy elements. Thief 3 dials back the steampunk, but still has less fantasy elements than the first game. The 2014 remake fails to capture the "thiefiness" of the series.