A particularly troublesome creature is encountered within the Cult of the White Shadow adventure. The Shadow Snake is that creature - it is a (thankfully) very rare creature that appears in the Valley of the Old Ones, some say spontaneously, but from the prolonged interaction of the material world with magics from the shadow dimensions. The process is rumor, and is not completely understood, but it at least offers up (however unlikely) an explanation for the presence of the Shadow Snake in the world of the Valley.
From the mad and gibbering descriptions gained from the occasional deranged Cultists that are actually captured alive, it appears as if the the infernal (or shadow) beings that the Cult is trying to summon to our world - the otherworldly and horrid Shadow Wyrms - have imitative lesser beings that the servants of shadow rely on for magical and other purposes. Those lesser beings are the Shadow Snakes.
A Shadow Wyrm appears as a vast shadowy body of a giant man-wyrm (snake like body, with pairs of clawed legs, at intervals, appearing out of the body; the front does not end with a simple head, but rather with a humanoid torso, with arms, wings out back, and a curious cobra-hooded head, with hollow black eyes). It is said that to look into the eyes of a Shadow Wyrm is to lose your soul. In imitation of these vast, horrific beings, a Shadow Snake is somewhat lesser, being man sized in the torso and head, and lacking wings and legs on the lower serpentine body. In fact, while in the world of the Valley, the Shadow Snakes appear to be (and are) quite solid and material from the torso up, but the serpentine body slowly fades to cloudy wisps of shadow.
When encountered as a hunting party that have come looking for victims for their masters, the Shadow Wyrms, the Shadow Snakes will seek out any intelligent warm blooded victim. They usually do not give preference to gender, race, profession or ability. (The exception to this, in the Cult of the White Shadow adventure is that preference will be given to anyone who encounters the curious orange fog).
An encountered party of Shadow Snakes will consist of some number of warriors (as per stats below), for every 10 warriors, there is a Champion (4 hit dice, 2 weapon attacks/round). For every 20 warriors, there will be two Champions and a Priest (level 7 cleric, servant of Lady of Air).
Num Appearing: 1d6, or as summoned or created.
Alignment: Chaotic Evil
Movement: 120' fly
Armor Class: 6
Hit Dice: 2hd (12hp)
Attacks: 1 weapon attack (shadow spear), 1 bite
Damage: 1d6+2 (shadow spear); 1d8 (bite)
Special: If the Shadow-Snake bites its victim, make a saving throw vs. poison. Failure means that the victim becomes allergic to light for the next 24 hours. Any light source brighter than a dim candle, or a small torch or lantern, will result in painful burns (1d6 damage) every round the victim remains in the light.
The Shadow Spear will do double damage if it strikes in the dark.
The Shadow-Snakes are semi-invisible at all times (and to all modes of site, unless in their home shadow dimension), meaning that those who rely on site in combat are at a -4 to hit them.
Within the Cult of the White Shadow, there are circulated rumors of specific spells used to contact or summon Shadow Snakes. Certainly, at one time, among the worshipers of the Lady of Air, there were such spells, and communication with the shadow dimension was quite common, as much so as among higher priests of the Westron Church communicating with the Astral plane.
The first such spell is known as the Shadow Clutch. It allows a single cleric, capable of casting a 4th level spell, to summon 1d6 Shadow Snake warriors, which will remain in the world, doing the bidding of their summoner (with appropriate bribes of blood from warm blooded intelligent victims), for a period of time up to 1 day per level of the spell caster. The true strength of the spell seems to be when it is simultaneously cast by up to five clerics (a group of five is referred to as a Clutch). With each additional caster, past the first, the number of dice of Shadow Snake warriors that are retrieved from the shadow dimension is doubled. So with two clerics, there are 2 dice; with three clerics there are 4 dice; with four clerics there are 8 dice; and with five clerics there are 16 dice worth of Shadow Snakes retrieved.
Shadow Clutch (spell)
Duration: 1 day per level of lowest cleric involved
Successfully casting this spell brings 1d6 Shadow Snake warriors from the Shadow Dimensions to a point within 30' of the caster. There may be multiple casters, up to five, each of which would have to know and simultaneously cast the spell. Each additional caster after the first doubles the number of dice worth of Shadow Snake warriors to retrieve. For every 10 warriors, there will be a Champion (4 hit dice, 2 attacks/round). For every 20 warriors, there will be a Priest (level 7 cleric, servant of Lady of Air), as well as two Champions. The casters must be within 10 feet of each while casting.
In addition to summoning warriors, it is also possible to subject a hated undividual to the Shadow Hunt, where they will be haunted by Shadow Snakes, their Champions and Priests, as well as Shadow Creatures - all come from the shadow dimensions in order to either destroy or retrieve the poor, unfortunate victim. This curse can only be lifted by a Remove Curse spell, or greater.
Shadow Hunt (spell)
Duration: Until removed
A victim cursed by the Shadow Hunt spell will begin to attract beings from the shadow dimensions, who come to the world of the Valley of the Old Ones, specifically to attack and kill (or drag back to the shadow dimensions, in the case of a Shadow Snake priest) the victim. At random intervals (determined by the DM, but should be at least once per week) new members of the hunt appear somewhere in the vicinity (within 1 mile) of the victim, and will have an uncanny sense as to the victim's location. The number of Shadow Snakes that arrive is equal to a dice toss of 4d6. Again, for every 10 warriors, there also appears 1 Champion. For every 20 warriors, there also appears 1 Priest, in addition to the 2 Champions. For ever 6 rolled on the dice, there also appears 1 Shadow Creature, serving the leader of the hunt (a Priest if available, if not then a Champion, or one of the Warriors).