Way of the Naga (Way of the Ascendant Dragon 5e Monk variant)
by Marc Saindon"Snakes...why'd it have to be snakes?"
Taken as is, Way of the Ascendant Dragon overlaps too much with (the much better) Dracology of Glantri, so for both theme and redundancy issues, I'll recycle the subclass and make it about Nagas instead.
The snakefolk aren't a character race in Mystara, but in-game there could be legends about them. It's up to the DM whether that primordial race of snakemen ever existed. Maybe they got displaced when non-magical Alphatians settled in Ochalea, literally forced underground (reminds me of both of the Sidhe and REH tales of the Picts). Maybe they were always underground: the Good Kingdom/Feywild has its Feydark (Feywild Underdark) under Ochalea called Nagaloka (a sort of "Cobra-La"), playing a role similar to the Formorian, a race of plotting schemers vowing revenge on the rulers above (rather than Eladrins, Ochalea's fanon Feywild "Kunlun" is ruled by Celestial Dragons and the 5 Elemental Courts). At some point, Nagas and human cultists interacted, and the Nagas decided to intervene in world events through intermediaries, training an elite group of agents into their martial arts (which resembles muay thai).
Way of the Naga Monks are usually Lawful Evil (a very few are "pragmatic LN"). The Ascendant Dragon powers lose their versatility, and the only option for the draconic damage type they can use is always Poison. As a counterbalance to this nerfing, third level of Monk also provides part of Purity of Body, granting immunity to Poison (but not disease, which is still at level 10): Way of the Naga Monks undergo a process of mithridatism, ingesting a mix of small doses of poison (see Princess Bride) to build up immunity when they reach level 3 (while it's not how it would work in real-life, this is a fantasy game).
Wings Unfurled is replaced with Serpentine Body, which fuses the Monk's legs into a snake tail, which the DM can homebrew certain abilities (a free Tail Lash attack, advantage on climbing surfaces, advantage on grappling through constriction, increased jumping distance, advantage on sweep attacks, etc.).
Way of the Naga Monks organize themselves into cell-like structures called Nests. They are often clandestine, as their work as assassins make them either criminals or assassins employed by rulers. They are adaptable, spreading beyond Ochalea, and integrating the local culture into their recruitment practices, so in Thothia they might be called the "Apepnu" or "Apophidians" (followers of Apophis), while in the Northern Reaches name themselves after the Midgard Serpent. Monks typically opt for a serpentine name like Solid Snake, Ophidia, Black Mamba, Copperhead, Serpentor, King Cobra (Trouser Snake has been banned). Snake tattoos are common among members to show devotion to their Cause, although despite this outward display of loyalty, Nests will often bicker among themselves as often as they fight their rivals over the "one true way" of doing things. The existence of the real Nagas masterminding it all is up to the DM, the only important thing is the belief in them.
"Cobra !"
Notes:
Nāga
Muay Thai
Mithridatism
(Image: https://www.deviantart.com/topsidejohnny20/art/Ni-Hao-Du-1012497942)