Saint Germain is the standing example of how an esoteric legend is built, because the historical man and the immortal adept can be separated cleanly and the seam is visible.
The historical man appears in the record from the 1740s: a courtier of unknown birth and excellent manners who moved through London, Paris, The Hague, St Petersburg and the German courts, spoke several languages well, played the violin and composed, dabbled in dyes and chemistry, and was employed by Louis XV on at least one diplomatic errand. He impressed Casanova and Horace Walpole, both of whom recorded that they could not work out where his money came from. He said things in company that implied great age. Whether he meant them seriously is not recoverable.
His death is documented as precisely as an eighteenth-century death can be. He died on 27 February 1784 at Eckernforde in Schleswig-Holstein, under the protection of Prince Charles of Hesse-Cassel; the burial on 2 March is entered in the register of St Nicolai church, the cost is entered in the church accounts, and on 3 April the town authorities advertised the auction of his effects for want of a claimant. The effects contained no gold, no jewels and no elixir, about eighty-two Reichsthalers and some clothes.
The immortal begins after that. He was reported alive at intervals through the nineteenth century, and the Theosophists then enrolled him permanently: Blavatsky counted him among the adepts, and the later Ascended Master teachings, particularly the "I AM" movement of the 1930s and its successors, made him a Master of the Violet Flame who directs a spiritual hierarchy and had previously been, among others, Francis Bacon. None of this has any evidential relationship to the man who was buried at Eckernforde. It is a religious development, and it should be described as one rather than as biography.