Pyrichiel is the creature that Adam has to fight after having solved the puzzles in the third test of the Key to the Abyss. In the official official Interplay he is identified as a major demon and Master of Fire, his name containing the Greek prefix πυρ (pyr) which means "fire".
Unsurprisingly then, he attacks Adam with what appears to be a flamethrower. We don't have to fight him and we're free to exit through the green arch, but defeating him yields us the Perdition Shield, which protects us from two powerful fire demons roaming around the Island of Threads, the stage that we enter immediately afterwards. If we choose to engage in a battle with him, he will call out some fierce and fearsome lines, one of which we can identify as a quotation from John Milton's epic poem Paradise Lost:
To bottomless perdition, there to dwell
Great Fire of London
According to the Timeline which has been published in issue 65 of PC Format magazine, Pyrichiel was summoned by Florentine in 1666 to cause the historically documented Great Fire of London.
Steganography by Johannes Trithemius
Pyrichiel also appears in the 24th chapter of the Steganographia, an occultist work encompassing three volumes, written by Johannes Trithemius in the 15th century and dealing mainly with magic-related subjects, such as using spirits to communicate over long distances. Lately, however, these writings have been known to be actually concerned with cryptography and steganography. Trithemius also invented the so-called tabula recta, a device of encryption.
Lemegeton Clavicula Salomonis
Pyrichiel is also described in the second part of The Lesser Key of Solomon or Lemegeton Clavicula Salomonis, an anonymous grimoire that has been compiled in the 17th century.
The fourth of these wandering princes in order is called Pirichiel, He hath no princes nor Dukes under him But knights: whereof we shall mention of them They being sufficient for practice who have servants under them, They are to be called according to ye planetary motion. They are all good by nature, and will doe your will willingly. Theire Names and Seales are as followeth. his eight Knights: Damarsiel, Cardiel, Almasor, Nemariel, Menariel, Demediel, Hursiel, Cuprisiel.
- Come and face me, Adam. Face the fire of your soul! It burns here. And it will burn you.
- To bottomless perdition there to dwell in adamantine chains and penal fire. What say you, Adam? 
- Do you hold a flame for her then, Adam? Or is the fire extinguished? She betrayed you, you know that, don't you?
- ↑ Johannes Trithemius, Steganographie: Ars per occultam Scripturam animi sui voluntatem absentibus aperiendi certu, Liber I, Capitulum XIV
- ↑ The Lesser Key of Solomon or Lemegeton Clavicula Salomonis, Second Part: Theurgia Goetia
- ↑ These words originate from John Milton's epic poem "Paradise Lost" (Book I, 47f).