Flux represents a chaotic overflow of magic. It is produced by various magical devices, and it is also the end-stage of the corruption in which Taint inflicts upon logs of wood.
Various Forms Edit
Flux comes in two forms, gaseous ("flux gas") and liquid ("flux goo"). Both will inflict the Taint status effect on contact, but the gas flows upwards while the liquid flows downwards. Like other fluids, both can also flow sideways to seek a hole, but unlike water or lava, they have constant volume, and do not expand indefinitely. Flux goo has the ability to destroy liquids below it when it falls, similarly to Liquid Death.
If flux gas reaches the open sky, it will dissipate harmlessly into the void, but (at least in the overworld), flux goo lacks that option. Both will dissipate eventually regardless, but if they remain in contact with a block for too long, they can spawn Fibrous Taint, thus tainting the block and changing its biome(its entire column from bedrock to sky has been tainted).
Both blocks are unaffected by tools, and cannot be scooped up in a bucket. However, they can be destroyed by placing another block in their space, either a solid block such as dirt, or a water source block. (Warning: Flowing water will hide flux without removing it.) A powered Flux Scrubber will also remove flux in the vicinity.
Not to be confused with "Redstone Flux", a type of energy used by several other mods.