So, as many of you will know, the purpose of the Ravenwing is to hunt down the traitorous Dark Angels that betrayed the Legion in the depths of the Horus Heresy. The Dark Angel successor chapters (Angels of Absolution, Angels of Redemption etc) broadly share this purpose and AFAIK they also maintain a Ravenwing company (although, possibly with a different name).

My question is, are there any other specific formations in Warhammer 40k lore that have a similar purpose as the Ravenwing (discounting the Grey Knights)?

Edit: Other than the Ravenwing, the only formation I can think of would be the White Scars "Huntmaster" type thing they have going on hunting down those who have damaged the chapter as in "The Hunt for Voldorius".