Segestes

Segestes was a noble of the Germanic tribe of the Cherusci involved in the events surrounding the Roman attempts to conquer northern Germany during the reign of Roman Emperor Augustus.
Arminius, the Cheruscan noble and military leader, had married Thusnelda, Segestes' daughter, against her father's will. Segestes, who favored Roman overlordship, bore Arminius a lifelong grudge and eventually found ways to exact his revenge. In 9 AD he warned the Roman governor Publius Quinctilius Varus of the impending uprising of his countrymen, but was not believed. Varus and his three legions subsequently perished in the three-day Battle of the Teutoburg Forest, where several allied German tribes under the command of Arminius ambushed them.
Segestes openly turned against Arminius when Germanicus invaded northern Germany in 15 AD in a renewed attempt to establish Roman rule in the area. Besieged in his stronghold by his own countrymen, Segestes appealed for help to Germanicus who relieved him, and Segestes then handed over his pregnant daughter Thusnelda, Arminius' wife, to Germanicus as a prisoner. Thusnelda was taken to Rome and, together with her brother Segimundus, displayed in Germanicus' victory parade in 17 AD, with her father an honored spectator. Thusnelda never returned to her homeland. Arminius' only son, Thumelicus, whom she bore while in captivity, was trained as a gladiator in Ravenna and died in a gladiator fight before reaching the age of 20.
The final blow was delivered in 21 AD when Segestes and other members of his family murdered Arminius. Segestes was eventually given a residence by Germanicus somewhere in the Roman provinces west of the Rhine.

• http://www.clades-variana.com, in German
• Thusnelda, part of the Encyclopædia Romana by James Grout.