The Hollow is an ancient and mysterious vapor that absorbs all magic it comes into contact with. For this reason, it is considered one of the most powerful forces in existence. The Hollow manifests itself as a swarm of black particles that seeks out magical power, though it possesses no true sentience.
In ancient times, the forces of good and evil worked together to defeat the Hollow, as it would consume and destroy all magic unless it was contained. Through the use of a spell, they managed to contain the Hollow inside a box and sealed it away within an ancient burial ground, where it would be guarded by both good and evil for all eternity.
History
The Source of All Evil
After his Oracle was vanquished, the Source of All Evil called upon the Seer to foresee his future. When she predicted his defeat at the hands of the Charmed Ones, the Source grew desperate and obtained The Hollow. He infected the Devil guarding the burial ground and sent him after the sisters to absorb their powers. He is then vanquished by Piper and her powers are absorbed by the Hollow.
The Source then sent a Darklighter after the sisters and the Hollow absorbed Paige's powers as well. With two of the three powerless, the Source decided to attack the sisters himself. The Seer then summoned the mortal Cole Turner and told him only he could save Phoebe by taking in the Hollow himself.
As the Source attacked the sisters, Cole intervened and absorbed his powers. The sisters then vanquished the Source with a spell and called back the Hollow by working with the Seer, who took it back to the burial ground.
The Ultimate Battle
In order to win the Ultimate Battle, the Triad decided that the Jenkins sisters needed to summon the Hollow to defeat the Charmed Ones. However, this was overheard by the demon Nomed, who informed the sisters as part of his attempt to get rid of his former masters. This led to both sets of sisters summoning the force at the same time, resulting in it splitting between them.
When the Charmed Ones and Billie and Christy confronted each other, their magic intertwined and caused a huge explosion, destroying the Halliwell Manor and leaving Billie and Piper as the only survivors. Piper later went back in time and managed to prevent this from happening by returning the Hollow to its burial ground through a spell with her mother and grandmother.