The Mergence is a group mind consciousness. It travels from planet to planet absorbing all life. It was made aware of Earth by the Coroner. It took the Mergence two hundred years to arrive to Earth.
The Mergence sent a part of it's mind in physical form ahead of itself in the form of Tenebrae. She was to make sure everything was ready for the Mergence's arrival. The Mergence needed to join with the Coroner in order to absorb the Earth.
It arrived, but the Silver Surfer attacked and caused doubt in the mind of the Coroner. The attempt to merge with him failed. This also separated Tenebrae from the group mind.
It gave the Coroner a second chance and they finally merged. The Mergence know set it's sights on killing Tenebrae, because if absorbed, she would corrupt the minds with individuality. Tenebrae and Psycho-Man attacked the Mergence with Psycho-Man's psycho-kinetic energy. This would have turned the minds inside the Mergence in on themselves and destroy them.
The Silver Surfer stopped this from happening. The Mergence spit out the Coroner and agreed to leave Earth alone.