The Battle of Abrittus (now Razgrad, Bulgaria), also known as the Battle of Forum Terebronii, occurred in 251 between the Roman legions and Goths under King Cniva. The Romans were defeated, and the Emperor Decius and his son Herennius were both killed during battle, largely thanks to the failure of the general Trebonianus Gallus to attack aggressively. Decius became the first Roman emperor killed in a battle with barbarians.
Gallus, who became emperor upon Decius' death, negotiated a treaty with the Goths under duress, which allowed them to keep their booty and return to their homes on the other side of the Danube, while at the same time promising an annual tribute in return for the Goths' promise to respect Roman territory.
Ammianus Marcellinus (31.5.12-17) rates this reverse with the most serious military disasters of the Roman Empire to his time: Varus' defeat at the Battle of the Teutoburg Forest, the incursions of the Marcomanni during the reign of Marcus Aurelius, and the Battle of Adrianople.