Fluxuations are pure wisps of instability that are created by chaotic or otherwise anarchic events. They can combine into a Flux Node if enough converge in a certain area.
Fluxuations are not inherently hostile, but are still very dangerous. It drains the health from any mob (or player) who walk within 15 blocks at a rate of half a heart per 1 second. They have 25 hearts of health. If you walk within 5 blocks of them, they will hit you with what looks like electrical arcs, which further increases the damage they deal by another 1.5 hearts. Luckily, they spawn very rarely, but drop diamonds with a 33% chance and drop Fluxium Alloy at a chance of 15%. There is a 10% chance to spawn Nidarc on death.