Short answers, pulled from the story.
Theosophy was founded primarily by the Russian Helena Blavatsky, along with Americans Henry Steel Olcott and William Quan Judge. The Theosophical Society was established in New York City in 1875, following a meeting of the Miracle Club on the 7th of September 1875.
In Theosophical belief, the Masters (also called Mahatmas, Adepts, or Elder Brothers) are a fraternity of spiritually advanced human men who possess supernatural powers including clairvoyance and the ability to project their soul to any location. Blavatsky said their chief residence was in Tibet and that they were the real founders of the modern Theosophical movement. The most prominent Masters in Theosophical literature are Koot Hoomi and Morya.
After Blavatsky's death in 1891, a schism developed over succession and the authenticity of messages claimed to come from the Masters. William Quan Judge led the American section to secede in April 1895, forming the Theosophical Society in America. The international Adyar-based Society was later led by Annie Besant, under whom membership grew to a peak of 40,000 in the late 1920s before declining after the Great Depression.
Theosophy teaches that the human spirit repeatedly incarnates into new human bodies on Earth, never into other life forms. After death, the soul passes through kama-loka (a state Blavatsky compared to limbo) and then into devachan (likened to paradise), remaining there for 1,000 to 1,500 years according to Blavatsky. Karma regulates this cycle, ensuring that actions in one life determine circumstances in the next.
Theosophy was directly formative for several pioneers of abstract art. Hilma af Klint developed abstraction through her work with the Theosophical Society. Wassily Kandinsky was influenced by Theosophical ideas about color, and Piet Mondrian drew on Theosophical symbolism. The composers Alexander Scriabin, Gustav Holst, Cyril Scott, and Edmund Rubbra were also shaped by Theosophical concerns.
Theosophy inspired over 100 esoteric movements and philosophies. Rudolf Steiner founded the Anthroposophical Society on the 28th of December 1912 after splitting from the Theosophists. Alice Bailey established the Arcane School in 1923. Theosophy also influenced the I AM group, the Church Universal and Triumphant, the New Age movement, and the Völkisch-derived occult system known as Ariosophy.