The Way of the Long Death in Jaibul
by Marc Saindonwon't re-invent the wheel with that one. These Monks live in Sind, although most are found in Jaibul ("team evil" city). Joining their group means you forsake your Caste, which marks you as an outsider from Sind society. The Way of the Long Death recognizes the gods, but see them as a level of power to be attained, not a deity to be worshipped (which they are right, as Mystara has Immortals, not gods, but they aren't verifiably sure of that, only have faith in the idea). A lot of their behavior is anti-social or frowned upon (see Aghoris), but they see it as transcending fear and shame through the odd discipline.
Their combat style is martial yoga (although they do also practice what we call normal "yoga" to meditate), the masters are called Yogi (instead of Sifu or Sensei). Lawful Evil is a common Alignment, although a few could just be "medieval metalheads" who enjoy the shock value but aren't too keen on actually harming others (see Black Metal Vegan Chef). I guess you could have Lawful Good Monks of the Long Death, tending leper colonies and helping outcasts, using their powers to alleviate pain from which there is no way to back from (which kind of cuts-and-pastes M:TA's Euthanatos into Mystara).
While all of this might seem cliché and resemble Temple of Doom thuggee cults, remember the pulp roots of Mystara. Not everything in 5e needs to be a sociology term paper.
Notes:
Aghori
(Image: https://www.deviantart.com/malkovicht1949/art/Dhalsim-Street-Fighter-1002397472)