Hans and Gretta Strauss were the names of a pair of dead siblings found in XV century Germany. They are the disguise of a powerful demon. In his little kids form, the demon was portrayed by Lindsay Taylor and Shawn Pyfrom while in his real form he was portrayed by Roger Morrisey.
The demon's disguise, which manifested every 50 years to create strife and tear communities apart, was speculated to be the basis of the fairy tale of Hansel and Gretel. The first record of their appearance was in the Black Forest of 17th century Germany.
The pair appeared as murdered children to Joyce Summers in Sunnydale. The children then haunted her in visions, encouraging her to "punish the bad girls." After Joyce forms Mothers Opposed to the Occult and the adults of Sunnydale are roused into an angry mob, trying to burn Buffy, Willow and Amy, Giles casts a spell revealing the demon's true corporeal form. Buffy then impales the demon through the neck with the stake she is tied to.