The Great Poisoning was an Irken attack on a certain area of the universe. This area included Serpentia. It is unknown who launched this attack.
The attack consisted of spraying and splattering chemicals over certain planets. This killed or sickened many of the planets' inhabitants, leaving the planets free to easily conquer.
Some chemicals affected the inhabitants. Nio was hatched during the Great Poisoning. The chemicals altered his development, causing him to have abnormally small wings. This affected him greatly, causing teasing from his peers. This is why he hates Irkens.