Here is the 4e text on Muls (from the Campaign Guide):
Muls are half-dwarves, descended from the union of a human and a dwarf. They have the stature, agility, and mental flexibility of humankind, coupled with the physical resilience and endurance of dwarves - a rare combination of qualities that makes muls more
than a simple blend of the two races.
Because they are strong, tough, quick, and blessed with fantastic endurance, muls are highly prized as slaves. In fact, most muls are born into slavery.
Play a mul if you want...
- to be tough as nails.
- to play a hero who fought his or her way out of the bonds of slavery.
- to be a member of a race that favors the fighter, barbarian, and battlemind classes.
. . .
Most muls begin their lives as slaves. Slaveholders throughout the Tyr Region have long known that tremendous hardiness and stamina result from mixing human and dwarven lines. Muls make outstanding gladiators, slave warriors, and heavy laborers, enduring toil and hardships that would kill lesser folk.
Muls who set their hearts on freedom are difficult to keep in chains. Some escape to the wilds and become raiders or join tribes of ex-slaves, whereas others who escape become mercenaries and sell their fighting skills to whomever they can. Muls who don’t flee captivity can win their freedom in the arena or by completing a dangerous task for their masters. A few highly prized gladiators receive so many privileges and comforts that they are effectively free, enjoying great latitude to go where they want and do as they wish. The Dungeon Master might have mul heroes start the campaign as slaves. If not, assume that your mul character has already won his or her freedom by the time the game begins.
Muls are hard, driven, pragmatic folk with little remorse or sympathy in their hearts. Many grow up under the lash, having been taken from their parents while very young and subjected to brutal training for the arena or grinding toil in fields or quarries. Consequently, muls have a hard time offering friendship and trust to anyone. More than a few muls, scarred by the hardships of their upbringing, spend their days as bitter, violent misanthropes. Others are suspicious, grasping mercenaries who have learned never to lift a finger on behalf of another person without establishing what they will gain from providing aid. Despite their tendency to be sullen or self-centered, muls can learn to work alongside others. Growing up in the slave pits and the underclass of society taught them how to forge alliances and understandings; their survival demanded nothing less. . . .
The word “mul” is derived from the Dwarven term mulzhennedar, which means “strength.” Pronunciation varies
throughout the Tyr Region; the word can be pronounced as mool, mull, or mule, although this last variation is considered derogatory and might start a fight. Given the derivation of the name, sages who care about such matters regard mull as the most accurate pronunciation.
I think it illustrates some of the problems that a 5e-era revival of Dark Sun would face.