The Gruenstalher Twins were a pair of terrorists who acted as antagonists in the Buffy episode "Homecoming" - they were two brothers who worked in union with their superior, the Old Man.
History
Prior to the events of Homecoming the Gruenstalher Twins were said to have engaged in numerous terrorist activities such as captial murder, terrorism and the bombing of Flight 1402 - they were seemingly of German origin as well since they were both wanted criminals in that country.
When Mr. Trick organised a SlayerFest '98 contest to see who could kill Buffy Summers and Faith Lehane the Gruenstalher Twins were more than willing to take part in the "fun" alongside the Old Man, who would act as their commander.
Despite being terrorists with experience in combat the Gruenstalher Twins would meet a rather embarassing end by shooting each other by mistake - putting an end to their terrorist careers once and for all.