Trellium-D was a substance used to protect ships from anomalies in the Delphic Expanse, but it was devastating to the Vulcan nervous system and became an addiction for T'Pol.
The Delphic Expanse was riddled with spatial anomalies, and trellium-D proved effective at insulating a ship's hull against them. Enterprise acquired the material during its mission there.
In its refined form, however, trellium-D disrupted Vulcan neural pathways. The crew of the Vulcan ship Seleya had been driven violently mad by exposure to it. T'Pol later used small amounts to unlock her emotions, became addicted, and suffered lasting damage before she gave it up.