The Vandenreich are a group of exceptionally powerful Quincy who somehow survived the genocide that Soul Society committed against them centuries prior the beginning of BLEACH. Their name in Japanese (見えざる帝国 (ヴァンデンライヒ), Vandenraihi) means "Invisible Empire". Under the orders of Emperor Yhwach, they conquered Hueco Mundo, imprisoning the new ruler Tia Harribel and using the remaining Arrancars living in Las Noches as cannon fodder for their war.
The Vandenreich’s strongest warriors go to form the ranks of the Stern Ritter elite, and each of them is identified by a letter and a battle name bestowed by the Emperor himself. Known Stern Ritters are:
The Jagdarmee (狩猟部隊 (ヤークトアルメー), Yākutoarumē) is the Hunting Squad of Hueco Mundo. Their mission is to capture Arrancars, find the strongest and most skilled ones and forcibly enroll them into their army to fight their war. Their leader, the First Jagdarmee Quilge Opie, is a strong enough warrior that the Emperor granted him a letter and a battle name like the Stern Ritter.
The Soldat (聖兵 (ゾルダート), Zorudāto)
The Vandenreich has subjugated several strong Arrancar from Hueco Mundo. They were drafted into the army of the Vandenreich and used as expendable fodder.