The Dacians were a group of tribes in the Thracian family who inhabited what is now Romania. They were briefly united under a king named Burebista in the 1st Century BCE; he achieved fame for driving the Celtic Boii out of what became his personal kingdom. The nobles of the the tribes murdered him because they feared he had grown too powerful - interestingly, they did this in 44 BCE.
After a century of tribal anarchy, Dacia was reunited under a chieftain named Decebalus, who subsequently waged war againt the Roman Empire. The Emperor Domitian paid him tribute, but in 101 CE the Emperor Trajan waged a war against Dacia. Decebalus surrendered the next year and was made a client king, with Roman officers overseeing the Emperor's will in Dacia.
In 105, Decebalus rose in revolt and initiated a bloody war against Rome. The next year his capital at Sarmizegetusa was destroyed, its people sold into slavery, and Decebalus took his own life while being pursued by the Roman cavalry. Though sporadic violence continued, Dacia became a heavily romanized province, until it was abandoned to the Goths by Aurelian in the early 270s.