A liquid that annihilates everything in a large radius when mixed with anything. Note that pouring it or throwing a phial with it on the ground counts as mixing with salt. It can be sold by ichor merchants and the Alchemist in Bethesda Susa. Also, the alchemist will often throw a phial of this around when engaging creatures, so clear the floor as soon as possible before talking to him.
When eaten, a meal cooked with neutron flux as manual ingredient will either give you a permanent +1 AV or instantly kill you. Dying in this way will grant the Starry Demise achievement. The odds for +1 AV and instant death are 4:1.