In their natural form, the Gwanzulum were large, monstrous heads with muscular, stubby arms and legs.
The Gwanzulum were telepaths who could read their targets' minds and take the form of individuals from their memories. The gwanzulum were parasitic energy leeches, draining life force from and ultimately killing their targets. (COMIC: Planet of the Dead)
During a conflict known as the Shaper Wars, the Gwanzulum all but wiped out the Whifferdills. The wars ended with the last of the Gwanzulum stranded on the planet Adeki. They drained the life force of the Adeki natives, and remained alone on the planet.
The Seventh Doctor encountered the Gwanzulum when he stopped on Adeki for a fishing trip. Reading the Doctor's mind and beginning to feed off his life-force, the Gwanzulum took the forms of the Doctor's past companions and selves to trick him into letting them aboard the TARDIS. The "companions" attempt was wrecked by taking the form of Peri, who was not actually dead, and fighting among themselves in a panic to get him to take them. The "Doctors" instead worked together, with the Seventh missing uncharacteristic moments in the confusion.
Horrified by carvings in a tunnel wall describing the Shaper Wars, the Doctor attempted to warn his "other selves" that the Gwanzulum wanted to enter the TARDIS to spread to new worlds. However, an uncharacteristic suggestion by his "Fourth self" to violently wipe out the monsters caused him to see through the Gwanzulum's deception and made his escape alone. (COMIC: Planet of the Dead)