The event was in 2016.
Proxima Centauri is just 4.2 light years away, but it's a member of the smallest class of normal stars called red dwarfs, which only emit faint visible light. As such, it sits at magnitude 11—whereas the human eye can see magnitudes up to 6 or 7. (A low magnitude number indicates a high brightness, and some solar system objects are bright enough to dip into negative numbers on the scale.) The flare event placed the star at magnitude 6.8, which would have been as bright as a dim star on a clear, very dark night.
The planet circling this star would have had 68x more solar energy dumped into its atmosphere. The possibility of life around this star just went from a maybe to a mostly not.
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demure: https://arxiv.org/abs/1804.02001 for those interested in reading the preprint.
We use these results and the Evryscope flare rates to model the photochemical effects of NOx atmospheric species generated by particle events from this extreme stellar activity, and show that the repeated flaring is sufficient to reduce the ozone of an Earth-like atmosphere by 90% within five years. We estimate complete depletion occurs within several hundred kyr. The UV light produced by the Evryscope superflare therefore reached the surface with ~100X the intensity required to kill simple UV-hardy microorganisms, suggesting that life would struggle to survive in the areas of Proxima b exposed to these flares.