Blight

The Blight is a strange sickness which appears in some oases in the Desert of Desolation. Plants grow warped and gain a twisted animation as they reach out to prey on animals passing by. Creatures mutate and develop strange growths as they grow to immense size. Insects in particular are vulnerable to this, with Giant Scorpions frequently appearing as a result of Blight exposure.
The source of the Blight is unknown, but when it appears at an oasis the first signs are an explosion of plant growth. Plants grow larger and stronger, then gain an aggressive vitality as they change to become predatory species. Much like the Dreamkiller Tree, the plants will adapt to hunt animals. Some turn into immense versions of the Venus Flytrap, snapping huge leaves closed around prey. Cacti may evolve into dart-shooters, expelling thorny spines coated with toxins, using puffs of air to fire the thorns. Other plants develop immensely sticky acidic sap which they will drip over creatures which take shelter beneath their leaves, immobilising the creature and slowly dissolving it to fertilise the plant.
Blight infestations swiftly spread to the animals drinking at the oasis. They will grow larger or warped at a great rate, jackals shedding fur to reveal muscles like hawsers underneath skin broken by oozing sores. Scorpions, dragonflies and the like grow to immense size, large enough to prey on humans. The animals and insects no longer feel pain or fear, and are vicious predators. Humans who drink repeatedly at an affected oasis soon start to show similar symptoms.
The sickness spreads from oasis to oasis through the underground aquifers. It is only possible to cleanse an oasis by killing and burning every affected creature and plant - in practice, destroying the entire oasis with intensely hot fire - and then teams of druids and clerics repeatedly casting cleansing rituals upon the ground and water. Some druids devote their lives to tracking down every emergence of the Blight, striving to trace it back to its origin. Other druids work to heal every afflicted oasis, restoring the life-giving water sources to the desert and regrowing the plant-life which provides food and shelter, holding back the march of the sands.