Information on Leg. XXI  Rapax
Legio XXI Rapax 

Bust of Octavian/Augustus as high priest. Museo Nacional de Arte Romano, Mérida. Legio XXI Rapax: one of the Roman legions. The name means 'predator'.
This legion was probably founded after 31 BCE by the emperor Augustus, who may have integrated older units into this new legion and added new recruits from northern Italy.

Its first assignment may have been in Hispania Tarraconensis, where it took part in Augustus' campaigns against the Cantabrians, which lasted from 25-13 BCE. However, the legion's stay in Hispania is nothing but a hypothesis. We have more certainty about its stay in Raetia, which was annexed in 15 by Augustus' son-in-law Tiberius (the future emperor). Its base was probably at Regina Castra, modern Regensburg.

Inscription from Philippi
In 6 CE, Tiberius was to lead at least eight legions (VIII Augusta  from Pannonia, XV Apollinaris and XX Valeria Victrix from Illyricum, XXI Rapax from Raetia, XIII Gemina, XIV Gemina and XVI Gallica from Germania Superior and an unknown unit) against king Maroboduus of the Marcomanni in Czechia. At the same time, I Germanica, V Alaudae, XVII, XVIII and XIX were to move against Maroboduus along the Elbe. It was to be the most grandiose operation that was ever conducted by a Roman army, but a rebellion in Pannonia obstructed its execution. XXIRapax was involved in its suppression.
After the disaster of Varus in the Teutoburg Forest (September 9 CE), where the legions XVII, XVIII and XIX were destroyed, the legion was redeployed in the province of Germania Inferior. It shared its base at Xanten with V Alaudae, keeping an eye on the nearby tribes of the Cugerni and Batavians, and guarding the confluence of the Rhine and Lippe. Both units took part in the Germanic campaigns of Germanicus in the first years of the reign of Tiberius (14-16; described here).

Vitellius  (Museo nazionale della civiltà romana, Roma; ©**)
In 21, a mixed subunit of XXI Rapax and XX Valeria Victrix, commanded by an officer from I Germanica, was sent out to suppress the rebellion of the Turoni in Gaul, who had revolted against the heavy Roman taxation under a nobleman named Julius Sacrovir and Julius Florus. Almost twenty years later, the Twenty-first was employed during the Germanic war of Caligula. The details, however, are not fully understood.
After Claudius' invasion of Britain in 43, XXI Rapax was redeployed in Germania Superior, which was now undergarrisoned. After a possible (but not proved) brief stay at Strasbourg, our unit was transferred to Vindonissa (modern Windisch in Switzerland), where it succeeded XIII Gemina. Here, it defended the passes across the Alps against a possible Germanic invasion of Italy. 

In 47, the soldiers rebuilt the fortress, which had been constructed out of wood, from natural stone and bricks. At Ruperswyl, they built kilns, where tiles and pottery were produced - not only for Windisch, but also for other military settlements in this area.