After your feedback on my Twisted Summoner, I thought I would run this subclass past you guys. Constructive feedback is welcome.
Holy Killers of the Eternal Secret
by JohnL73Background
These are a combination of Islamic Assassins and the Medjai (chaps from Brendan Frasier's Mummy movies who were determined to keep Imhotep and Hamunaptura a secret). They are devotees of the Eternal Truth who are fanatical about keeping Nithia as secret as possible. They are a variant of thieves and prefer mundane skills to magical illusion to stay hidden until they strike.
Founded long before the rise of Al-Kadim and the spread of the Eternal Truth, they are the most dedicated opponents of ancient Nithia and anyone who tries to bring ancient Nithia back to Mystara. The Holy Killers' most common targets are treasure hunters seeking to unearth forbidden Nithian artefacts to sell to unscrupulous dealers, both native Ylari and those from other nations looking to make their fortune. However, a sinister new enemy has emerged, Nithian cultists intent on resurrecting their destroyed religion.
There are those who have drifted away from the true faith, and now sell their skills to the highest bidder. The holy killers might be in the service of the greater good but they have very marketable skills, and being motivated by the prophet can easily turn into profit motive.
Game Rules
For these holy killers I feel like going with the AD&D version of Backstabbing (from 1E AD&D PHB):
Back stabbing is the striking of a blow from behind, be it with club, dagger, or sword. The damage done per hit is twice normal for the weapon used per four experience levels of the thief, i.e. double damage at levels 1-4, triple at 5-8, quadruple at levels 9-12, and quintuple at levels 13-16. Note that striking by surprise from behind also increases the hit probability by 20% (+4 on the thief’s “to hit” die roll).
Holy Killers like B/X thieves can use any type of weapon. But they can only backstab while using a normal sword, shortsword or dagger as their back stabbing technique involves thrusting the blade between certain ribs, which cannot be done with axes, maces or clubs. Backstabbing against non-humanoids (such as animals, magical beasts, constructs and undead) only does double damage as anatomy is not the same.
Holy killers can and sometimes do use poison, either ingested or insinuated on a blade, though they do not get any special bonuses when using it.
Holy killers of the Eternal Secret can use most of the standard thief skills. However, they have swapped the thief's ability to find and disarm traps with the new ability to disguise themselves. This is not the same as impersonating a specific individual. but can either be used to look like a specific job, role or the like, such as a palace guard, a merchant or a housewife (base 10% + 3% per level) or simply not appear as the holy killer normally does, so that those who know the holy killer's face are less likely to recognise him (base 15%, +5% per level). Situational adjustments may be applied by the DM (e.g. if in a crowd the holy killer might get a bonus, if being stopped and questioned by a sergeant of the town watch there may be a penalty. Similarly time spent practicing a role and observing those he tries to imitate will give a bonus (noticing how palace guards salute and march, or the accents people in a particular town use).