Umbranth
An umbranth is an undesirable entity that exists somewhere between AulCelestian reality and the outer-universe. Their only goal seems to be to reek havoc, destroying anything in site.
Though they are typically prevented from entering AulCeles, they are able to create projections through which to interact with its inhabitants. Through interaction with these projections, creatures capable of manipulating mana can inadvertedly allow the Umbranth to manifest. These manifestations are never permanent but can result in large-scale destruction.
Prevention
The presence of umbranths is well documented by all tribes of AulCeles and various methods exist to protect a settlement from penetration. The main prevention method is to teach children to avoid their projections - to never go near them and to avoid even looking at them.
During the 'Dark Month'
A period of time in the year known as the 'Dark Month', during which all natural forms of light are ineffective and even mana-based light struggles. At this time, the seals that prevent umbranths from crossing into AulCelestian reality are weakened, allowing stronger umbranths to pass through.
Although incapable of manifesting due to poor visibility, the threat then occurs when the Dark Month ends and tribes find themselves surrounded by huge shadows.