Umbra-Arcana is a supposedly fictional type of arcana detailed in the history of the goddess Saldr's religious canon. It is death and destruction manifest, a pool of it said to be in place of her covered eye that she and she alone can draw from to even destroy the bright realms.

"Her covered left eye is often a point of contention. Some say it was lost in a fight, others remind them that only Rhongomyniad can harm a God, and the only injury that was not outright death in all recorded history fell on Demi’kes, who himself had lost an eye. Others claim a different story entirely - her eye is not only there, but acts as a pool of mythical and divine destructive energy, a fictitious type of magic referred to as ‘Umbra-Arcana.' - the Long-Song. This sect of religious canon claims that upon lifting her eye guard, that swirling pool of Arcana can be drawn from. She could theoretically encompass her blade in it, and to strike anything with it would mean its total death and/or destruction. It is also said that even looking at something with purpose and intensity would atomise it, reducing it to dust. The final legend of this theorised Arcana is that she can elect to destroy the entirety of the Bright Realms, reducing it to nothing but space. The only thing it cannot affect in this religious canon is a fellow God. Not even the Arcane Anthem is safe from it - Saldr may force the choir to halt its singing, bringing the symphony to a stop."

(via Saldr's Article)