Holy Killers of the Eternal Secret

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Background

 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.

The Holy Killers' current base is a fortress high in the hills near the border mountains where they train and receive their orders, known as the Citadel of the Bleeding Blade, referred to by members as "the Citadel" . They are led by the mysterious Old Man of the Hills, a cleric of considerable power and charisma who  can convince recruits of the importance and sincerity of the mission so well that many become willing to die for his cause. These recruits come from the urchins, orphans and unwanted children of slaves in the various towns and cities around the Emirates, offered food, shelter, purpose in life and maybe excitement and adventure. 

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. Some of these are loners but in Ylaruam city itself there is a shadowy group of them calling themselves the Guild of Assassins. They are even more paranoid and ruthless than the holy killer of the Eternal Secret, and have the dreaded mantra "Nothing is True, Everything is Permitted". 

The Bead of Oblivion only affects a 24-mile hex outside Surra-Man-Raa. This is only a small part of the Emirate of Nithia. Anyone approaching Nithia from another direction (such as Cinsa-Man-Noo or Ylaruam City) won't be anywhere near the bead. The Order of the Holy Killers are aware of the Bead of Oblivion's effect (if not the bead itself), and most stay away from the area outside Surra-Man-Raa. Even if they do enter it (either knowingly to chase a target or accidentally), they only forget about Nithia, not that they are members of the sect and what their target is (though they may magically forget why they are hunting the target). They can always be re-educated as necessary once back at their base. In fact, some of them may not even need to be told the details about ancient Nithia, just that tomb-robbing in the hills of Nithia is a crime against Al-Kalim and the immortals because it risks unleashing terrible evils.

The Holy Killers of the Eternal Truth are a backup if the tomb-robbing intruders avoid the area of effect of the Bead of Oblivion. Immortals may send supernatural agents to cause certain followers to forget about Nithia. But the Holy Killers are a more thorough, less gentle way of dealing with those who have avoided the Bead of Oblivion. And they believe they are doing the work of the Immortals and of Al-Kalim (which they are, in a bloody sort of way).

Game Rules

For these holy killers I am 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).

Ostensibly Holy Killers can be any alignment though most are neutral - they may be killers but they target those who they believe are threats or deserve to die. Some however are chaotic, and enjoy killing too much. These are more likely to abandon the order and join the guild of assassins. 

Image by Zvonimir Grbasic (source)