Rifts in the Web

Rifts - not the ones in the ground, but tears in the Web of Magic.  They come in two classes, inter-planar and intra-planar.  Some call them planar gates, but whichever name you pick, they are holes in the Web of Magic.  They come about from use of magic in an area for a prolonged time, like what happened in the Gulimbor Catacysm.  While others come about from the use of magic items or casters opening them.  In the Dawn War, the primordials used these rifts to cross great distances across the planes. or between worlds  Those that opposed the primoridals, the gods and angels of the Throndar and Bal-Kriav.  Sarseg used this gate to send his hordes against a region of Brucrumus known to hold a hospital and training ground for the angelic armies.  Sarseg was crafty when he created the Radullu Rift.  He knew quite a bit about the workings of the Web of Magic - thanks to once being an untainted weave spider.  When Sarseq built the Radullu Rift, he surrounded it with an area of  dead magic which made it irreparable. 

mane preparing to become one with a Manes Chimney

Dawn Era, actively sought to close them as soon as they were detected, or risk being overrun by a seemingly endless horde of primordial soldiers.

The Radullu Rift would lead to great changes in the topography of  Brucrumus's western fringe.  The area on Bal-Kriav's side became flooded from a lake on Throndar.  The water of this lake still feeds into this realm from high atop a plateau of the Northern Hordelands.  The tremendous flow of water created Radullu, Orukhan, and led to the creation of a vast swamp south of the area - a region of the realm now known as the Troll Bogs.   The Nature Protectorate Boglin says that the lake on Throndar would surely have drained by now, but for another gateway somewhere out their that itself acts like a drain from the Troll Bogs, or somewhere else, and forces water back into Throndar's lake.

One of the longest non-permanent gates to remain open was the Sôvuk Sea Gate.  In the Demon Spawn War, this gate was opened by the demon lord Gorag'theg.  He and his abyssal horde used it to cross the planes, leaving the Abyss and entering Bal-Kriav.  The weave spiders took so long to close this gate because Gorag'theg built the Manes Chimneys.  These ghastly towers burned soul energy and in turn provided a counter to the repairing being done by the weave spiders.