Brokk

 was a Grold. After stealing genetic modification drugs from the Semquess, they chased him and caused him to crash on the island of Salutua on Earth in the 1880s. He drove the natives away and set up a distortion field around the island from the Semquess. While exploring the island, he damaged the heat exchanger of his environment suit, causing him to go dormant.

When Marshal Grover's expedition arrived in 1934, Brokk managed to hypnotise David Ferraro and make the man build a fire to revive him.

Recently prior to this, the Semquess had arrived and were searching the island for the drugs. They attacked the reanimated Brokk, who was holding Nancy Norton hostage. Because the Third Doctor had created a time bridge to the island from UNIT in the future, several potential timelines were created. (PROSE: The Eye of the Giant)

Original timeline
The Semquess destroy Brokk and his ship, causing a volcanic eruption that kills everyone on the island. One of the shp fragment, saturated with omicron radiation, wound up at UNIT. The Doctor used it to create the time bridge.

Alternate timeline 1
The UNIT troops sent to find the Doctor attack the Semquess to protect Nancy. This gave Brokk a chance to make a deal with her. He gave her his eyebrain, then sent his mindless body to pilot his ship. The Semquess blew it up, thinking they had killed Brokk and destroyed the drugs.

Nancy used one of the drug ampoules to merge a part of the eyebrain with her. This gave her hypnotic powers which she used to take over the Earth, with her as its goddess. However, the time bridge was still open, and the new timeline not fully actualized. The Doctor went back to the island to avert the changes.

Alternate timeline 2
The future Nancy and her minions followed the Doctor back to 1934. Realizing he could not reason with her, he used the sonic screwdriver to shatter the eyebrain fragments, killing her. This also killed David Ferraro and Tilly, among others, who had fragments in their heads which Nancy had been using to control them.

Unfortunately, before she died, she fell on another Semquess drug ampoule. It broke and caused her to merge with the fragments and the Constitution, the Marshall Grover expedition's ship. The insane creation proved unstoppable, and destroyed everything on Earth.

Alternate timeline 3
Amelia Grover consumed the drug from the last ampoule. She died and was resurrected as an angelic being. She stopped the ship-creature, and left Earth with it, promising to separate Nancy and Brokk when she could figure out how to do so safely. This timeline became the new permanent one.