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5e realized that skulks needed something more than just being "the invisible man". So they gave them stonking big claws.

Skulks are a race of evil humanoids within the realms of Dungeons & Dragons, known to inhabit the settings of Greyhawk, the Forgotten Realms, and the Nentir Vale, skulks resemble hairless humans, but have the magical ability to blend into their background like a cartoon chameleon, making them effectively invisible. So, basically an alternate form of Doppelganger.

Oerthian Skulks[edit]

The original form of the race, skulks were the creation of one Simon Muth, who submitted them to White Dwarf and saw them officialized in the Fiend Folio. In this initial presentation, they were described as human cowards whose cowardice had eventually resulted in them magically developing the ability to become virtually invisible. Resentful, spiteful bastards, these skulks pursued a parasitic life as squatters, thieves, and murderers.

When updated to 2nd edition in the Greyhawk Monstrous Compendium Appendix, skulks had... literally not changed a bit.

The first change in skulk lore came when they received a writeup in the Dragon Magazine article "Legacies of the Suel Imperium", which postulated that skulks might have either been created as assassins by the Suel Imperium, or else might be connected to the Suloise Goddess of Treachery and Deception, Syrul. They were portrayed with a slightly greater bit of nuance, but still the same cowardly thieving murderers.

The final depiction of skulks came in first the 3e Fiend Folio, and then in the 3.5 Races of Destiny. Here, at last, they received a new backstory as members of an underprivileged caste within a lost empire whose resentment ultimately led to them using powerful magic to draw power from their "untouchable" status, transforming them into a race of human chameleons who then went on a murderous rampage against their oppressors and who basically haven't stopped since.

Faerunian Skulks[edit]

Faerun developed its own population of skulks in "Faiths & Avatars". Here, they were originally a tribe of nomadic humans who had wandered Calimshan after the defeat of the Djen. They were abducted by drow and forced into servitude in the city of Guallidurth for centuries, their descendants kept passive by their fear of the darkness surrounding them. They finally escaped when a giant lizard appeared from the darkness, drove off the drow overseers, and led the slaves away. The lizard was an avatar of Ibrandul known as the Stalker and through it, the humans learned his worship.

While some of these men and women returned to the surface to spread Ibrandul's faith, the rest remained in the Underdark, surviving by raiding the farms and caravans of Guallidurth as well as those of human settlements on the surface. Over time (and continued use of a particular spell provided by their god) these people became the known as skulks and could thereafter be found living near many entrances to the Underdark.

Nerathian Skulks[edit]

The Nerathian Skulk appeared in the Monster Manual 3 for Dungeons & Dragons 4th edition. These skulks were originally a caste of slaves in Bael Turath - and when the nobles of that city signed the pact with Asmodeus that turned them into the first tieflings, these slaves were exempted from being considered part of the agreement. Outraged, the proto-skulks responded with a massive empire-wide riot, slaughtering many of their former masters over a single night of carnage. This attracted the attention of Graz'zt, who was so delighted by their treachery and murderous instincts that he signed a pact of his own. This compact, recorded in an Abyssal manuscript called the Tome of the Blinded Eye, forever bound the skulks to the cult of the Dark Prince, but in return imbued them with their powers of stealth and murder.

These skulks still serve Graz'zt faithfully, but their focused enmity remains on the other scions of Bael Turath; they prey on tieflings above all other victims, and have done their best to ensure that Bael Turath can never rise again in any form. Tieflings who achieve renown invariably attract the bitter attentions of skulk assassins.

Skulk PCs[edit]

Skulk PCs have existed, despite their... rather lackluster fluff. AD&D stats appeared in the above-mentioned Dragon #241 article, whilst 3e presented skulks as race in Races of Destiny.

AD&D Stats[edit]

Ability Score Minimum/Maximum: Strength 3/18, Dexterity 6/18, Constitution 3/18, Intelligence 3/18, Wisdom 3/16, Charisma 3/14
Ability Score Adjustments: +2 Dexterity, -1 Strength, -1 Wisdom, -4 Charisma
Class & Level Restrictions: Fighter 2, Cleric 2, Mage 4, Illusionist 6, Thief Unlimited
Multiclass Options: Fighter/Thief, Cleric/Thief, Thief/Mage, Thief/Illusionist
Thieving Skill Racial Adjustments: Pick Pockets -20% , Find/Remove Traps +5%, Move Silently +3)%, Hide in Shadows +85%
Natural Armor Class: 10
Movement Base: 12
An unclothed skulk can blend into its background with a combination of color-changed skin and absolute stillness, allowing even non-thief skulks to use their 85% Hide in Shadows base chance.
Skulks that move in silence impose a -3 penalty to their opponents' surprise rolls.
Even a non-thief skulk can backstab like a thief.
In a forest or underground setting, those attempting to use Tracking to find a skulk reduce their Wisdom to 1/3rd its value (rounding down) for the purpose of the proficiency roll.
Skulks cannot camouflage themselves if wearing armor, helmets, shields or clothing. This strips them of their bonus to Hide in Shadows.
Skulks are normally nocturnal, and at the DM's decision may take a -1 penalty to attack rolls & saving throws if forced to operate during the day, until they have adjusteds to a diurnal cycle by doing so for 5d6 days.
Skulks are affected normally by Charm Person and Hold Person. Magical items that affect humans alone have a 50% chance of affecting skulks.
Skulks suffer a -6 penalty to saves vs. fear, and if they fail their save have a 50/50 chance to either be frozen in fear or to flee in terror.
At the DM's discretion, a skulk that manages to become completely fearless may break whatever magical ability creates a skulk, turning irrevocably into a human.
Weapon Proficiencies: Dagger, Short Sword, Hand Axe

3e Stats[edit]

+4 Dexterity, -2 Wisdom, -4 Charisma
Humanoid (Human)
Base land speed 30 feet
Racial Hit Dice: A skulk begins with two levels of humanoid, which provide 2d8 Hit Dice, a base attack bonus of +1, and base saving throw bonuses of Fort +0, Ref +3, and Will +0.
Racial Skills: A skulk’s humanoid levels give it skill points equal to 5 × (2 + Int modifier). Its class skills are Hide (Dex) and Move Silently (Dex).
Racial Feats: A skulk’s humanoid levels give it one feat.
+8 racial bonus on Move Silently checks and a +15 racial bonus on Hide checks: Skulks excel at keeping to the shadows. These bonuses apply only when a skulk is wearing light armor or no armor.
Innate Nondetection (Su): Skulks are difficult to detect by divination spells such as clairaudience/clairvoyance, locate creature, detect spells, and items such as a crystal ball. If a divination is attempted upon a skulk, the caster of the divination must succeed on a DC 20 caster level check.
Peerless Camouflage (Ex): Skulks can move at full speed (and can even run) while hiding, taking no penalties on Hide checks due to movement.
Trackless Path (Ex): The DC of any Survival check to follow the trail of a skulk is increased by 10.
Automatic Language: Common. Bonus Languages: Dwarven, Elven, Gnome, Goblin, Halfling, and Orc.
Favored Class: Rogue.
Level adjustment +1.

Alternatively, a Skulk can function using the Monster Class rules. In this case, a skulk's base racial stats are:

+2 Dexterity, -2 Wisdom, -4 Charisma
Humanoid (Human)
Base speed 30 feet
+4 racial bonus on Move Silently, +5 racial bonus on Hide, both only apply when wearing light armor or no armor.

The class functions as follows:

1st Level: BAB +0, HD 1d8, Fort Save +0, Ref Save +2, Will Save +0, Skill Points (2+Int mod) X 4, CR 1/2, Feat
2nd Level: BAB +1, HD 2d8, Fort Save +0, Ref Save +3, Will Save +0, Skill Points 2 + Int Mod, Trackless Path, +2 Hide, +5 Move Silently
3rd Level: BAB +1, HD 2d8, Fort Save +0, Ref Save +3, Will Save +0, +2 Dexterity, Innate Nondetection, Peerless Camouflage, +2 Hide, +5 Move Silently

The skulk's class skills are Hide and Move Silently, and its racial class grants proficiency with all simple weapons but not with any armor or shield.