NOTE: I have stopped working on the mod. The most recent update/snapshot is on MCreator. So yeah.

The Darksteel Mod is a mod primarily focused towards the adventuring aspect of Minecraft. As well as adding a subtle story element.

Content

The Darksteel Mod currently contains

~25 items
~85 blocks
13 entities
~9 hostile mobs
3 "ambient" entities
2 minibosses
1 boss
3 new types of stone
A new ore set
2 biomes
9 structures (not including trees and such)
A mini-dungeon that can only be found with commands
16 advancements
A dungeon building kit
118 recipes

Dungeon Building Kit

The Darksteel Mod comes with a creative tab of items helpful for dungeon building. These include:

Key
Used for unlocking Lock Blocks. Cannot be crafted.
Using it will destroy the key.
Lock Block
An unbreakable block that can be removed with the use of a key, removing it from your inventory.
Meant to be used to block certain areas until the player obtains a key.
Boss Key
Used for unlocking Boss Lock Blocks. Cannot be crafted.
Using it will destroy the key.
Boss Lock Block
Like a Lock Block but more intimidating and uses Boss Keys.
Meant to be used at the end of a dungeon, where a boss would be located.
Redstone Lock Block
Similar to a standard Lock Block, but it is only removed with a redstone signal.
Lock Block Chain
Breaks when a neighboring block changes and a player is in survival or adventure mode.
This is usually used in situations where the only possible block change is a type of Lock.
It's main purpose to make a single Lock Block block off a larger gap than a single block.
Dungeon Curse
Changes all players in survival mode that are in a 42 block radius switch to adventure mode, and all outside adventure players back into survival.
This is used to prevent cheating, while not creating some annoying unbreakable chunk of area.
Fragile Dungeon Curse
Has same effect as Dungeon Curse. However, you dont need an item to remove it. You just need to reach it to undo the curse.
Curse Key
A key that drops from the boss. Using it can get rid of the Dungeon Curse, allowing you to break and place near the dungeon.
Using it will destroy the key.
Boss Spawnblocks
Currently can spawn the Wither, the Darksteel Berserker, the Darksteel Brute, the Giant Brillfly, and an Elder Guardian. These act like Lock Block Chains, but spawns it's respective boss/miniboss when broken.
More will be added in future updates.
Surprise Block
A block that will do one of 20 random things (or has ~30% to do nothing and vanish) if the world is in normal or hard difficulty.
Note that the block can spawn entities, and edit blocks directly below it. This block is mainly used to reduce repetition a little.

Items and Blocks

There are a fair amount of items and blocks to consider in the Darksteel Mod.

Ores and Metals:
The main way of getting Darksteel is through killing Darksteel based enemies. These enemies drop Darksteel Nuggets, and rarely Darksteel Ingots.
Darksteel tools and armor are one tier above diamond in the same way diamond is one tier above iron. However, Darksteel is much less durable than diamond.
Darksteel can also be obtained from chests and such, like other ores.
Darksteel can also be used to make darksteel pillars and darksteel walls. If you ever end up with too high a quantity. Darksteel Pillars and Walls can be found as parts of structures, but they can't be reverted into ingots.

There are also Lightpearls which are an ore that spawns near the surface of the Ruins biome. Lightpearl Ore drops Lightpearl Dust, which can be crafted into Lightpearls, which can then be crafted into Lightpearl Blocks. Lightpearl Dust is used to make Light Bullets, the ammo needed for the Lightsprayer.
1 Lightpearl Dust is also used in the Darksteel Nugget to Darksteel Ingot recipe. having it the middle with 8 Darksteel Nuggets surrounding it.

Stones:
Ruinstone is the primary building block used in the Ruins, as well as the rock beneath the grass. It can be crafted into Ruinstone Bricks, Polished Ruinstone, and Chiseled Ruinstone Bricks. Combining Chiseled Ruinstone Bricks with a Lightpearl will grant a glowing Chiseled Ruinstone Bricks.

Gravtite is a brown stone located in teh Deathly Woods. It can be crafted into Polished Gravtite, Gravtite Bricks, and from there Chiseled Gravtite.

Lemonstone is a plated stone that can spawn in sandy areas, such as beaches and deserts. It has a brick, smooth, polished, and chiseled variants, similarly to Ruinstone and other such stones

All stones have stairs, slabs, and walls.

Wood:
Only one wood set currently exists, Deathly Wood.
It is found in the Deathly Woods and can be crafted into most the standard wooden blocks.

Misc:
Vases can be crafted with a 7 clay. Bottom two rows filled completely and the top only in the middle.
They can be colored to red, blue, green, or purple, using the respective dye and a blank vase.
They can be used to store small amounts of storage, and will spawn with loot in structures.

Darksteel Knight Heads and it's variants are purely decorative blocks that rarely drop from Darksteel Knights and other such variants. Unlike most heads, they cannot be worn (yet).

The Fractured Sundial is a powerful tool that skips shortly ahead in time whenever activated with redstone. Best used in quick succession.

Entites:

The Darksteel Mod as 6 entities currently.

Dutent:
The Dutent is a passive mammal native to the Ruins. It drops Dutent Ham.

Darksteel Drone:
Darksteel Drones are weak flying machines controlled through light manipulation. They drop darksteel nuggets and do not spawn in the world naturally.

Darksteel Troops:
Darksteel Troops consist of Darksteel Knights, Darksteel Lightsprayers, Darksteel Spearmen, and Darksteel Warriors. Each having alterations in design and attack methods.
They drop darksteel nuggets, bars, uncommonly ingots, rarely their respective weapon, and very rarely, their heads.

Darksteel Brute:
A high ranked troop of the Darksteel Empire that follows under the rule of the Darksteel Berserker.
He drops darksteel materials as well as his damage increasing item called "Brute Force".

Darksteel Berserker:
One of the generals of Darksteel Empire stationed at the Time Temple to conduct studies of time manipulation.
He drops several Nuggets, bars, and Ingots, a Curse Key, and his Fractured Sundial.

Giant Brillfly:

An adult brillfly. An uncommon sight to be sure.
It drops a brillgem, which can be used to make a brillaltar. Which can then be given lightpearls to give night vision.

Ambient:
A few ambient entities, dont have much use.
But they make it look better so they're welcome.

The Darksteel Wiki:

The Darksteel Mod now has a wiki. I apologize that it's mostly uncompleted, it's still a work in progress.
It contains crafting recipes, mob stats, some known bugs, random trivia, all the good stuff.
<s>Except there's still a few missing pages but that's not important right now</s>

Click here or the header thing to go to it.

Notice:
The version mentioned throughout the description and images is still in beta.
So keep that in mind