Fell Goblin

Appearance:

As per the normal goblin phenotype with two exceptions. Fell Goblins have horns, typically like a Nubian goat, and are a bit more hirsute than the average goblin. The second is that their skin tone is not always the common base green or base brown. There have been reports of fell goblin tribes displaying red, light tan, yellow, gray, or even blue-colored skin. This is not to say that a blue fell goblin is a smurf, but more a dirty dark blue than any sort of electric blue.

Abilities:

As per normal goblins (insert whichever goblin template desired, the upgrade is the same across the board) but whatever the base, hit die are increased by 2, or its overall threat level is increased by 2. They are stronger, smarter, and more capable than normal goblins, but they are not super-powered goblins.

Damage resistance - minor across the board. Fell goblin leaders and heroes can have much higher levels of damage resistance, making them a serious threat to low-level adventurer-upon-return. This ability to survive low levels of injury allows for fell goblins to be more aggressive and adventurous themselves, so they will have higher instances of heroic goblins, mounted goblins, and goblins who branch into classes that most grots don't have the lifespan or patience to attain.

Lifespan - fell goblins can live up to a century

Elemental Resistance - as per damage resistance, fell goblins have a stronger base resistance to fire damage. They can be burned, but it takes deliberate effort.

Poison Immunity - fell goblins cannot be poisoned.

Smite - once per day, a fell goblin can call a smite attack against a lawful or good opponent and deal a greater degree of damage. In a swarm situation, a small number of fell goblins can swing above their weight class and deal serious damage to an unsuspecting foe.

Spell Resistance - As Fell Goblins increase in power, they gain a small amount of spell resistance.

Origins

Fell Goblins are the offspring of a female goblin and a male infernal creature. As goblins have litters rather than a single child at once, an entire tribe of fell goblins can be raised in a relatively short amount of time.

Possible Explanations:

The BBEG has summoned a handful of lower-level infernal beings for the sole purpose of breeding fell goblins to serve him, creating a fell goblin force similar to Saruman raising the Uruk-Hai from the orcs. In this instance, a few visual traits from the specific sort of infernal being summoned can be added to the template, so it becomes obvious that each tribe is functionally created by a single infernal creature.

The Goblins have summoned a fiend for the purpose of creating a new tribe of super goblins. This is typically going to be a more desperate act, possibly from a tribe that has lost most if not all of its fighting age males and faces being completely wiped out, or assimilated into another tribe, where the old tribe is dismantled and taken almost like loot.

In a really weird Romeo and Juliet tale, a goblin and a fiend have fallen in lust with each other, and the result is litters of fell goblins. This can lead to degeneracy as the fiend would have no reason to not mate with it's own female offspring, line breeding more infernal goblins.

Author's Note: Fell Goblins are only created by male infernal/female goblin crossings. A male goblin/female infernal will create functionally a tiefling, and it will be a goblinized version of whatever its mother is. On a second note, when creating fell goblins from infernal creatures, the infernal creature cannot be more than one size step above a goblin, so if for some sadistic hentai reason a Balor/Balrog demon decided to create a tribe of fell Balor goblins, it would fail because the size difference would kill the female either right out, or she would not survive pregnancy long enough to bring anything viable to term.

Usage:

Building Better Minions - fell goblins are a basic upgrade for grots to keep them viable in a game because of power level creep. They can gain classes, they can use black magic, they can swarm, and they're smarter than the average gobbo.

Introduction of Infernal - rather than throwing hamatulas, glabreezus, spatulas, and maraliths at players early on, fell goblins can be a way to beef up the powers of hell without going full Dante's Inferno right out of the gate.

Dark Hero Goblin - being able to stand on a level field with PC races, a fell goblin could be played with the Drizzt angle, overcoming goblin and infernal nature to become a hero. A somewhat dark and evil hero, but a hero all the same.