ADOM is a rogue-like game, wherein you play one of the many adventurers heading into the Drakalor chain to find the source of the corruption that is spreading throughout the land. In theory. In reality, what you're going to do is die. All the time. Sometimes before you get a chance to say a word to anyone.
This is because ADOM is one of the hardest, cruelest, most impossible games I've ever had the pleasure of playing. It's infinite in its complexity, and you're likely to spend months, maybe even years before managing to complete it. Personally, I've been playing it for about 4-5 years, and I've finished the game twice.
There's a multitude of ways the game can be played. You can plays as a fighter, mage, thief, necromancer, bard, ranger, archer, healer, monk and an elementalist, just to name a few. There's literally thousands of potential combinations for you to play as.
You move through the game world as @. At the most basic, you kill things by running into them and don't kill them by not. You talk with C when people are nearby. You go down stairs with > and you can go up them with <.
The game does a good job of conveying meaning without graphics. For example, a veteran may snigger at a blue g, but stare in horror at a room filled with black W's. I know I would! Each type of item has it's own character, such as rings being =, potions being !, Clothes and hats being ]. Color plays a major part as well, with special items being different colors and allowing you to easily discern them from one another.
The story can be as complex or simple as you make it. You can immerse yourself in it, or you can go through the entire game without saying a word to anyone (a little tricky but possible). It's good as a device for moving the story, but around your 100th character you start to skip it.
And 100 characters will just fly by. Personally I would have had literal thousands of characters, and they've almost all died horrible, sometimes really ironic deaths. The game revels in you dying. Every step is potentially your last, and at times you're going to hate the game.
But it all ends up worth it, somehow. Those few moments where all of a sudden the game just opens up and you become almost a demi-god, walking around and smiting all the letters of the alphabet with nary a care in the world are worth one hundred unfair stone block traps to the back of the head.
All up, it's staggeringly in-depth. To the point where if I wanted to write all that I could it would be a book, and if I wanted to write everything that I should to set you up for the game, it'd be about 50 pages long, in size 2 writing. I can't recommend this enough, but if you don't have a lot of spare time, you might have trouble getting anything done.