Despite hybrid eggs normally being destroyed due to being against the Silurians' sacred views of eugenics, Baal, Sula and Tahni's eggs were not, and their parents were not executed because of an imminent cataclysm was predicted. However, they had a shortened lifespan and were sterile, and sought to find a way to cure this condition after awakening from hibernation many millions of years later. To do this, they kidnapped a human teenager, Marc Marshall, and experimented on him, intending to use his DNA to cure their condition.
When Marc's presence was no longer useful to the hybrids, Tahni helped the Third Doctor escape with Marc to Smallmarshes but were captured by the Pale Man's security guards on the beach. Tahni was then taken to the Vault, but was freed when UNIT raided the facility. After Auggi's Silurian fleet failed to invade the British mainland, Baal and Tahni took command of the Silurian shelter. (PROSE: The Scales of Injustice)