Tenebrium Crystal and a Death Knight
Tenebrium is black mineral that appears and plays a major part in Divinity: Original Sin. It also makes a minor appearance in Divinity: Original Sin 2 in the form of a Tenebrium chest during the quest The Secrets of Bloodmoon Island. It is also the material used to build the Tomb of Lucian in Arx.
Divinity: Original Sin[]
The player first hears about Tenebrium in the mining town of Silverglen, where people have died of a disease called Rot after working in a nearby mine to extract Tenebrium. The mineral plays an important part in multiple quests, including The Naked Truth, The Troll's Bounty, and the main storyline quest A Source Hunter's Journey.
Mechanically, having a player character pick up any Tenebrium item (such as Tenebrium ore or a Tenebrium weapon) will cause the character to contract 
Rot1 non-leathal Void Damage per Turn , the worst disease in the game. While Rot causes the character to take 1 damage per second and cannot be healed by conventional means, the disease won't actually kill player characters, instead bringing them down to just one hitpoint.
The disease can only be cured by coming into contact with a Blood Stone, which will also heal all the player characters of all ailments and restore their hitpoints. Picking up a Blood Stone and using it from the inventory will have the same effect.
There are two ways of handling Tenebrium without contracting Rot:
- The first and easiest method is accepting a quest the The Troll's Bounty quest from Brandon in Silverglen. The quest involves transporting Tenebrium, and for this purpose Brandon gives you the Blood Stone Cage, a container that allows you to safely carry Tenebrium. Any Tenebrium items a character picks up will automatically be placed inside the Cage, shielding them from its harmful effects. If a character other than the one currently holding the Cage in their inventory picks up a Tenebrium item, the Blood Stone Cage is automatically moved into their inventory and the Tenebrium item is placed inside the Cage.
- The second method is acquiring the Tenebrium ability, which will make all the player characters permanently able to handle Tenebrium without suffering its ill effects. The ability can be acquired by either completing the aforementioned The Troll's Bounty quest, or by reading the book How to Handle Tenebrium. Unlike the Blood Stone Cage, this ability also allows characters to wield and use Tenebrium-infused weapons, provided that the character possesses the Tenebrium ability level required by the weapon. The Tenebrium ability is classified as a Weapon ability, and each point of the ability increases damage done by Tenebrium weapons by 10%.
Tenebrium weapons are the same as their normal metal or wooden counterparts, but do more damage. Also, it seems that you cannot improve Tenebrium weapons on a sharpening wheel.
The Conduit Leandra uses this rock to make Death Knights.
Even though the Void Dragon and the Trife have relatively low resistance against Tenebrium, attacking them with it actually heals them, so it is unwise to use Tenebrium weapons against them.