The Ancient One (The Curse of Fenric)

Ingiger (or Ingiga), better known as the Ancient One, was the last of the original Haemovores, after all of its children had died from the pollution.

Biography
Ingiger lived on a polluted Earth in the year 500,000, before it was brought to 10th century Transylvania by Fenric. Fenric commanded Ingiger to follow the Vikings who had stolen the flask containing him. Ingiger fed on them, turning many of them into Haemovores. It and the Haemovores followed the Vikings, (TV: The Curse of Fenric) chasing them to an island in the North Atlantic where the Twelfth Doctor, Vikings and Ice Warriors battled the Flood. The Doctor and Bill Potts encountered the Ancient One with the Doctor recognising him from their encounter in the Doctor's personal past. The Doctor realised that Fenric intended to replace the Haemovores with the Flood and convinced one to detonate a volcano to stop the Flood. Ingiger fled the island with his people and continued his hunt for the Vikings. (COMIC: The Wolves of Winter) After the surviving Vikings were killed by the pirate Hemming, Ingiger followed him to Maiden's Bay. Though Hemming and all his men were killed by the Ancient One, Hemming had buried the flask beneath St. Jude's Church, where the Haemovores could not get it.

Eventually, in 1943, the Ancient One was awakened again by Fenric. It was used to control the Haemovores, which were used to attack the soldiers in the base. Using the poisons made by the nearby base, Fenric intended to pollute the world, causing the future that created the Haemovores. The Seventh Doctor was able to convince Ingiger to betray Fenric, convincing it that Fenric would not return it to its own time. The Ancient One killed both itself and Fenric by going into the isolation chamber with Fenric and releasing a poison. (TV: The Curse of Fenric)

When Fenric recovered, he used a new Ancient One as his pawn, created from a Persian king. He was begged by the Doctor to release him from Fenric's chains but wouldn't. He went on a search for the Doctor and found his son Hurmzid. Leading the hordes of Haemovores against Weyland's Saxon, he was killed by a spear. Fenric seemed to have some remorse at the Ancient One's death. (AUDIO: Gods and Monsters)

Behind the scenes
The novelisation of The Curse of Fenric names the Ancient One "Ingiger" and gives its gender as female, despite being portrayed by a male actor.