|Being this Metal in a setting that doesn't have metal? That's pretty Metal.|
Originating from the Dark Sun Campaign Setting, Muls are the results of a forced breeding program, instituted by the tyrannical Sorcerer Kings. They are a great example of "hybrid vigour" - they are taller and more physically powerful than Dwarves, and tougher than either Dwarves or Humans. On average, they can go 72 hours without sleep, can march for days without rest, and possess a great, innate ability at combat. As such, they're mostly found in the Gladiatorial Pits, or out on the front lines as powerful soldiers and enforcers. They also see some use as slaves, particularly in heavy physical labour, due to their immense constitution.
I love Dark Sun - it's a setting where the usual expectations are turned on their heads, and the Muls are no exception. While Half-Elves and Half-Orcs have some of the features (and drawbacks) of both their parent races, Muls are almost completely separate from both - but draw on humanity's endurance and Dwarven toughness to make something altogether better. It's not something frequently seen in gaming, and for that, I like it.
Back in AD&D, they were natural Fighters - strong, tough, and deadly. They also made a reappearance in 4e, through the Dark Sun Campaign Setting book - and there, they were unstoppable. With bonuses to Endurance, Strength and Constitution, as well as the ability to shake off any lasting Conditions, they make fearsome Defenders, Fighters in particular, able to outlast many other races in taking punches, and laughing off any effects that might hinder them.
They're good enough that I try and drop them into other fantasy settings, with little or no changes to their lore or abilities. As a man who loves to dick about with a race's backstory, Muls still stand out to me - they're simply unnatural crossbreeds of Human and Dwarf, and still as tough as a brick wall.