Tanarukks are a species of Planetouched originating from the Forgotten Realms and one of the many "demihuman variants" of pre-existing Planetouched. Specifically, Tanarukks are the Orcish equivalents of Tieflings.
Tanarukks first appeared in 3e as part of the Races of Faerun splat, alongside their fellow "demihuman tieflings", the elf/succubus hybrids called Fey'ri. Their basic backstory is that, 500 years before the present, demons that had been summoned to the material plane conquered a keep in Faerun and took control of it, leading to it being named "Hellgate Keep". From there, they enslaved local orc tribes to serve them, eventually undertaking a breeding program to create a superior strain of fiend-blooded orcs to serve as their slaves. The result were the Tanarukks, a combination of some of the nastiest parts of both races. They had a Level Adjustment of +3, mostly due to starting with 5d8 worth of racial hit dice, and aside from being even stronger and tougher than orcs, were as slow as dwarves but had automatic proficiency with all martial weapons and the ability to cast Control Flame as a Sorcerer of equal character level 1/day.
Tanarukks went on to make a second appearance in the 4th edition's Monster Manual 3. Here, the "created by demons" aspect was given a respin; 4e tanarukks are literally created by demons and demon-worshipping evil mages, especially those allied to Baphomet. Vats filled with a pulsating morass of orc meat and demon ichor are subjected to blasphemous rituals, which causes them to heave forth dozens of tanarukks at a time. In contrast to their original 3.5 version, which were supposed to be better soldiers, 4th edition tanarukks were instead "orcs taken up to eleven": battle-crazed, vicious killing machines that live only for the slaughter. Their fluff even states that the first thing a tanarukk experiences upon being "born" is a solid blow to the head to keep it from immediately flying into a murderous rampage.
These Tanarukks are literally nothing more than a kind of fleshy golem; they live only to kill, being incapable of breeding or doing anything more productive. As they're high Paragon tier opponents (17th and 18th level) and have an "Indomitable Rampage" ability that lets them survive their first killing blow and gain attack & damage boosts from it, the fluff isn't joking around when it says a lone tanarukk can wipe out entire towns (remember: generic NPCs are presumed to be level 0) before someone strong enough to kill them finally takes them down. In fact, they actually relish the challenge of fighting legitimate opponents, which is one of the reasons why they rampage.
There are three kinds of 4e Tanarukk; the Marauder (Level 17 Skirmisher), the Flame Speaker (Level 17 Artillery) and the Steel Warrior (Level 18 Soldier). The "control fire" aspect from 3.5 was toned down with this edition; aside from the Flame Speaker, whose attacks revolve around throwing around fire like it's going out of style, the Marauder and the Steel Warrior only have one fiery attack each. Marauders have the Flame Harrower attack, where they ignite their axe, hit someone with it, and set them on fire. Steel Warriors likewise can channel fire along their weapon to hurt people, but they create Burning Cages and can technically do so from afar by shooting exploding fiery crossbow bolts.
Tanarukks would later reappear in 5th edition's "Volo's Guide to Monsters". Here, they were portrayed as more generally showing up whenever orcs either turned to demon worshipping themselves or were taken over by a demon-allied black mage; tanarukks, in this edition, are orcs mutated into fiendish versions of themselves in the womb. In this case, however, the Tanarukks' obsession with strength and sheer ferocity means even pureblood orcs are wary of the damn things, and standard procedure is to keep them locked and chained up away from the rest of the tribe when they don't need them. If they get free, their first instinct is to bloodily rampage through the tribe, trying to beat them all into submission so that they can take over. This is bad news for the orcs because Tanarukks are even more warlike and bloodthirsty then their pureblooded kin, usually leading to a conquered tribe being wiped out. As the tanarukk taint can breed true, any tanarukk offspring are usually killed at birth unless the tribe foresees a need for their strength.
5e's Tanarukks are explicitly linked to Baphomet, and have lost their fiery powers entirely. Instead, they're magic resistant, and have the ability to immediately make a bonus retaliatory attack with Advantage whenever somebody hits them in melee.
Also, both versions of the post-3.5 tanarukk gained a much better movement rate, shooting up to the standard 30 feet. Why the 3.5 version had a 20ft movement speed in the first place, who can say?
A different take on the Tanarukk in 3e, alongside a Fey'ri.