Toxicium is the 200th element in the periodic table, it is so unstable and acidic that almost no matter can withstand it.
If heated enough, it will begin glowing blue and transform into a violent gas, spreading out in all directions, turning all matter around it into Toxicium as well. Once this state of matter is achieved, Whatever universe it is present in will be 100% devoured in (omniversal) days, leaving only this blue, erasive gas.
The Blue Void is made of Toxicium in its gas state, making the Primordialverse even more difficult to access. Only the gravitational effects of Negative Black Holes can be used to counteract the "infectious" effect of Toxicium.
It is also extremely similar to Strange Matter in the way that it interacts with other materials, to a lesser extent.