Star Trek Expanded Universe

Little is known of the Entity. It is known to have made an appearance in the Delta Quadrant only once, when it destroyed the USS Dauntless and Deep Space Delta, and destabilized the Delta Wormhole, trapping what remained of the Starfleet's Delta Fleet in the F'asa Sector.

It is widely believed that the Entity was an energy cloud that feeds on emotion. It appears to affect all humanoids, causing individuals to relive periods of heightened emotion. In addition, the Entity has been known to cause deep coma in particular individuals. Once such occasion resulted in the long term incapacity of Captain Thomas Halloway of the USS Hunter in 2373. Halloway, currently being cared for in a medical facility on New Terra, has remained in a coma since his encounter with the Entity.

After its encounter with the Hunter the Entity traveled through the Delta Wormhole to the Alpha Quadrant, where it ravaged an entire fleet before disappearing in a flash of white light. With no trace of the Entity left, Starfleet categorized it as destroyed.