Planar Blisters

Perhaps the primary threat of the wilderness and the most impactful legacy of the Illithid Invasion, Planar Blisters are zones of dimensional overlap that can form almost anywhere. Mind Flayer travel to Khorvaire combined with their rapid retreat and various weapons they utilized destabilized the fabric of space-time, causing it to ripple and fold and, because of this, now other planes sometimes intrude upon Khorvaire, pushing through dimensional boundaries like ulcers. Though these rifts are usually quickly dispelled by magic users in cities while they are still small, out in the wilderness they can grow to immense size until they overlap massive areas of land. When they do, the land becomes warped and meshed with the nature of the plane intruding, bringing with it the flora and fauna (such as they are) of the foreign dimension. What's worse, the membrane over the expanding space between dimensions is so weak that it allows effortlessly travel for creatures to and from it, allowing outsider creatures to move into the material plane at will.
After they reach more than several meters in radius, a planar blister expands beyond the overlapped nexus that spawned it, forcing any that would wish to seal it to traverse the intervening (and spreading) planar area. As such, the sealing of these wilderness rifts has come to heavily rely upon adventurer & mercenary forces capable of combating the denizens of the intruding plane, traversing to the blisters center and resealing the nexus to collapse the blister back on itself. If unsealed, a planar blister can threaten life in a massive area and, as such, it is in the best interest of a land to do what it can to make sure they do not spread.