The Thel-ghan'che is an energetic, rather than physical, parasite that feeds on the good thoughts and intentions of a sentient creature, and enforces the carry-out of the evil ones. It is a brutal creature- it was the sole cause of all of the horrors committed by Valderis Robless during the many centuries for which he has been infected with one of the creatures. This parasite was removed by Eris-Tam and she became infected by it, which is the reason for the leaders' current mission, their attempt to cure her.
The First Infestation was the largest breakout of these creatures, and took place mainly on the Kazesqui-Alteriian homeworld. The Kazesqui discovered a way to cure their Alteriian Neiede's of the parasite by removing it, but after they did so there was a fifty-fifty chance that it would overpower and infect them instead. There was no such cure for the infected Kazesqui. At first, they were all held in cells built to keep them in, but soon space ran out to continue to hold all of them. The council of leaders was forced to make a tough decision- an order that all infected Kazesqui must be killed on sight. Thousands died, slaughtered by their own people, and thousands more were left broken without their Neiedes. It was one of the most devastating times in K-A history, second only to the Battle at Snowdove.
“This I choose to do. If there is a price, this I choose to pay. If it is my death, then I choose to die. Where this takes me, there I choose to go. I choose. This I choose to do.”
― Terry Pratchett