Shadowfell

At the beginning of the Great God Wars, there was a strong surge of necrotic energy that caused great deals of trouble for the residents of the Material Plane. To combat this, the gods convinced Luna to take the energy and create her own plane where the energy could be stored. This plane quickly became overwhelmed with spirits of the dead killed in combat by paladins who took up arms in the name of Luna.

However, with the betrayal of of the god Roma, Luna was imprisoned and separated from her domain. This void of power in the plane resulted in the formation of the Dark Powers, the spirits of the paladins that served Luna. The Powers took up the task of finding someone to rule rule over the Shadowfell. This led to a series of battles between the Dark Powers, leading to no true end for centuries.

This all changed when Powell, a powerful necromancer, displayed the ability to summon forth even the most dense necrotic energy. The Dark Powers then used their abilities to capture the soul of Powell upon his demise, and bestowed upon him the title of a Shadow Lord. As the Shadow Lord, Powell established the Obelisks of Dread, which serve as monuments that provide the Powers and their Shadow Lord appointees with the ability to creep into the Material Plane for small amounts of time depending on their following.

Since then, few beings have displayed a great power allowing them to be appointed to the position of Shadow Lord, but one who has shined greater than others was Elustria, who took up the mantle of the Raven Queen. As the Raven Queen, Elustria sought vengeance on the Material Plane, even more than those who came before her. In order to do this, she has gained the favor of several Dark Powers and worked closely with the Shadar-Kai, who she has sent out into the Material Plane to do her bidding in the hopes of one day restoring her physical form.

The Shadowfell has a potent affect on those who are not acclimated to its overwhelming necrotic energy. One such condition is the Corruption of Death, which can cause the most noble heroes to succumb to temptation. This works by forcing those non-native to the plane to make a DC 15 Wisdom Saving Throw every 24 hours they spend in the plane. With the first failed save, there are no permanent changes to an individual but they will begin to feel thoughts rising that maybe embracing the darkness of the Shadowfell will bring them the power they need to complete their task. After three failed saves, the individual will hear the voice of Luna in their mind telling them to sacrifice the thing they care most about in order to receive a boon from her. The individual will then need to make the Wisdom Saving throw every hour, upon a fail they will officially become corrupted and have to obey the wishes of Luna. If the individual is removed from the Shadowfell within three days of becoming corrupted, they can be returned to their original state of mind, otherwise they will fall to their corruption, dying and releasing their soul into the hands of the Dark Powers.