- He wrote sometime before 1135 'Prophetiae Merlini' or 'the prophecies of Merlin. This is a series of apocalyptic predictions that were made by the wizard Merlin, formerly known as the Myrddin written by Monmouth's Historia Regum Britanniae from before 1139.
- The history of the British kings is Geoffrey's famous book. The book describes the history of the Kings of Britain before the Saxon period. Many stories are just based on legends. The Historia is one of the first texts in which King Arthur occurs.
- His last known book was Vita Merlini: written sometime between 1149 and 1151. In it he tells the Merlin legend as it was known from the Welsh tradition.
The other historian who wrote in the 12th century about Merlin was Gerald of Wales, also known as Gerallt Gymro in Welsh or Giraldus Cambrensis. He was a medieval cleric and chronicler.
Merlin is named in his "Descriptio Cambriae.
It is already very difficult to find a dux bellorum as Arthur Pendragon in history. Arthur was probably a non-Christian leader of an army of Brits who beat around the year 500AD the Saxons at Mont Badonis.
- The history that is closest at the time there was' the Excidio et Conquestu Britanniae, a work from the 6th century of spiritual Gildas. But he says nothing at all about one Arthur, probably because this is a blatant infidel, let alone that he mentions the magician or Druid and Bard Merlin or Myrddin Emrys.
- Still, there are assumptions that Arthur Pendragon has really lived, built on the luttle details of the Battle of Mount Badon and the fact that quite a few babies in that time were not the modern name Arthur. Where was Arthur, Merlin was close.
- We can therefore establish reasonably safe Merlin who lived around the year 500.
- Gerald of Wales called Merlin's name in his Itinerarium Cambriae. This is due to the fact that Geoffrey of Monmouth, the Welsh name Myrddin Emrys translated in the name Merlinus Ambrose.
- In his Historia Regum Brittonum Merlin is mentioned as the son of Ambrosius Aurelianus. Ambrose was the leader of Britain and a descendant of the patrician Roman family Aurelii. He won according to Gildas and the legends of the Historia Britonum important battles against the Saxons in the 5th century. According to the Chronicon Maiora Ambrose came to power in 479.
- According to Geoffrey of Monmouth Ambrosius was the son of Constantine III, who was Roman emperor short time. As the eldest son, Constans, was killed on the orders of Vortigern, the two remaining young brothers, Ambrosius and Uther Pendragon were smuggled to England. Eventually, Constantine became King of Britain, succeeding Gracianus Municeps who was murdered.
- Anyway, assuming that Merlin's father was the commander and King Aurelius Ambrosius already prevailed by a single author bv Mary Steward, The Crystal Cave.