Argentium is a thick, metallic silver liquid that is similar in appearance to mercury and which originates from Asgard. The substance is extremely lethal to both mortals and witches. In the Witches of East End television series, Argentium seeped through the cracks in the portal that was located in the catacombs underneath the Fair Haven estate. It was revealed that Ramus Mortium, also known as the Branch of Death, contains small amounts of Argentium, which is partially what causes it to be so dangerous. When a person (either witch or mortal) is injected with Argentium, the substance attacks the brain, causing side effects such as seizures and internal bleeding that become progressively worse over time. If it's not treated with a special potion, it will be fatal to whomever has touched or ingested it.
However, it is apparently possible for a witch to build up an immunity to Argentium, as Frederick Beauchamp was tortured with small doses of Argentium by his grandfather, King Nikolaus, and as a result can survive exposure to the substance, although doing so requires him to sleep off the effects for several hours in order to heal. He demonstrated this immunity when he transferred Joanna's Argentium poisoning to himself to save her life.