Indigo Island
Indigo Island was named such for the dangerous plague that was discovered on the island hundreds of years ago, the Indigo Sickness. Magi, scholars, and healers have somewhat determined that the disease was created by a great Slann known as Zhul-Xotec, who was experimenting with biological weapons and dangerous poisons and chemicals.
The conclusion by most historians is that either something went arwy because of the Slann's hubris, or more likely, by interference by Tzeentch during the War of Arcanum. Regardless of what happened, pandora's box was opened and the last thing Zhul-Xotec could do before succumbing to one of the many plagues and biologically enhanced monstrosities was activate the islands automated security systems in order to stem the spread of most of the islands most parasitic and dangerous beings who could threaten the rest of Ardicil.