Thief: Deadly Shadows is a first person perspective Stealth action adventure game set in an atmospheric medieval dark fantasy world.
(Mostly Medieval though, it's not balls to the wall Elf people fantasy.)
You take the role of Garrett, a master thief.
He steals stuff.
In-between missions you can explore the city and complete side quests and screw around, all with amazing atmosphere and lighting.
Thief: Deadly Shadows uses the same modified Unreal Engine 2 that Deus Ex: Invisible War used. (Possibly an updated version)
It sports a Shadow Volume based lighting engine and a physics engine, so you can have shadow casting floppy ragdolls.
Thief: Deadly shadows has one of the scariest missions in gaming history, the Shalebridge Cradle.
Best not to spoil it.

I mean holy s*** I've seen it in quite a few Top 10 lists. It's literally an Asylum and an Orphanage. One step away from being an abandoned Hospital...
Anyways, Thief: Deadly Shadows is a stealth game, you peek around corners and complete your objectives without being seen.
If you are seen, you can engage in combat or run and hide so they return to their patrols.
There's a light gem on the bottom of the screen that tells you how lit you are, you generally want this to be dark.
Stick to the shadows.
They've added a new feature called "Wall Hug" where you press R (default) to flatten yourself against the wall, this is great for hiding in shadows casted by something thin and against the wall.
Garrett can also jump and climb, and later in the game you can purchase climbing gloves and climb like spiderman.
Garrett is also equipped with a bow and has various types of arrows he can use to help him on his missions.
-Broadhead Arrows (Standard Combat-Oriented Arrow)
-Water Arrows (Can dowse torches thus making areas dark)
-Moss Arrows (They create patches of flora that silent Garrett's footsteps when walked upon.
-Gas Arrows (They knock people out)
-Fire Arrows (They create light when aiming and do lots of damage, and also set oil patches on fire.)
-Noise Arrows (When fired they make a firework noise attracting enemies to where it was last heard.)
There are also a variety of items Garrett can use
-Flashbomb (Flashbang, blinds enemies so garrett can hide with ease. Quick getaway)
-Oil Flask (Thrown glass flask of Oil, people slip on the patches of oil like it's Home Alone.)
-Gas Bomb (Like Gas Arrows, they knock people out cold.)
-Holy Water (Primarily under utilized, only comes in handy on the Abysmal Gale.)
-Explosive Mine (Lethal Trap)
Garrett can also pick locks, and you can purchase practice locks from shops to memorize the sweet spots.
YouTube
You guys gotta play this game, it's so fun.