Seeker Squad
Not to be confused with the Sisters of Silence Seeker Cadre.
Seeker Squads or sometimes known as Seeker Space Marines or Headhunters were a type of specialist Space Marine infantry squad used during the Great Crusade and Horus Heresy. They were the assassins of the Space Marine Legions; the Alpha Legion has a absolute hard-on for these units as a result.
Overview
Their principal role on the battlefield was to identify an enemy command structure and destroy them with well-placed surgical strikes while the battle rages. It is said that the Alpha Legion first perfected these specialists and with the Emperor's approval spread them to the rest of the Legio Astartes.
These troops are chosen on the basis of pure merit, usually those in a Seeker squad are the best shots of their respective Companies. To aid with their kill, they are usually equipped with specialized Bolter ammunition to better deal with their chosen victims. The most famous example of these was when a group of Alpha Legion Seekers disguised themselves as Smurfs and almost assassinated Robby Blue in his pajamas. Due to their nature, Seeker Squads don't have a standard form of armoury; taking any weapon to suit their needs (Although the Nemesis Bolter seems to be their favourite) and even donning different power armour to disguise themselves as is the case in the Alpha Legion.
Crunch
Headhunters meant to take out enemy command elements; these are your Sternguard vets: 5-10 strong with Special-Issue ammo that is different from 40k, Implacable Advance, BS5 and Preferred Enemy against one enemy unit or Independent Character (their target), chosen after any infiltrators have been placed.
While their benefit against the chosen unit is obvious, you can still use them against pretty much anything, as the correct choice of specialist ammo and high accuracy can function against virtually anything except mid to heavy vehicles. Can still be equipped with combi-weapons just like Sternguard, though this will inevitably make the unit cost a small fortune in points and, if what you want is lots of special weapons, that´s what Tactical Support squads are for.
Lots of guys rapid firing Tempest bolts will annoy the crap out of your opponent; even if they are only S3, having to sit through your opponent placing 20 small templates with low scatter will drive you nuts, whilst the Shredding AP2 Scorpius bolts make them into a perfect sniper squad provided you can stay still and Kraken bolts give them a Rapid Fire range of 15" (and AP4 to mess with anyone not a MEQ). The icing on the cake - they have Precision Shot to allocate wounds on Apothecaries and whatnot. All these traits can make the unit appear to be a greater threat than it actually is.