I've written a lot about Headhunters since they are the inspiration for my Alpha Legion army. However, much as I would like it not to be true, in 40k an army of only "Headhunters", in the form of Sternguard, Noise Marines, and/or Chosen, will not do well. However, there were other units used by the Alpha Legion as infiltrators, saboteurs and assassins. Units tasked with such activities were all referred to as "Effrit", though this term most often referred to Headhunters. The term effrit comes from a kind of jinn, an ifrit, in Middle Eastern stories known for being intelligent, cunning, and ruthless.

Example depiction of an ifrit, or "effrit" in 40k (source).
One example of a unit specifically called out as Effrit in book 3 of the Forge World Horus Heresy are Destroyers. Destroyers were equipped with weaponry that only they and certain Mechanicum factions were allowed to use, forbidden to all others. This included radiological, biological, and alchemical weapons, such as rad missiles and phosphex bombs. Destroyers typically suffered the effects of the weapons they wielded, including cancers, pale skin, and lesions. They were often seen as dishonorable and disgraceful but a sometimes necessary evil in many legions. In the Alpha Legion, however, Destroyers were not shunned or looked down upon given the philosophy of the 20th that the ends justify the means. Since Destroyers are not a unit in the Space Marine or Chaos Space Marine books, they will be represented by Devastators or Havocs, or perhaps Deathwatch with suspensor-equipped heavy weapons when they get a real codex.






