Reiver

We have Sly Marbo Marines now! Piss yourself Chaos scum!

"All Space Marines are experts of the rapid strike, but Reivers take these skills to the next level. They are specially trained and equipped to launch brutally effective and unexpected assaults. They do this with a sudden fury and shocking violence, for the Reivers fulfil the role of terror troops that sow fear and confusion amongst their foes."

– Games Workshop

"Boo"

– A Reiver

Our Super Super Marines have Catachan Jungle Fighters in power armor now. A Reiver, who is usually part of a Reiver Squad, is a new type of combat specialist unique to formations of Primaris Space Marines who is tasked for rapid infiltration and lightning assaults. Where the Intercessors are the defensive battle line heavy infantry of the Primaris Astartes, the Reivers are elite assault specialists given special training and wargear. The Raven Guard and White Scars are known to favor their use to supplement their own fast attack doctrines.

The Reiver squad is armed with over-sized combat knives glimmering with disruption fields, heavy bolt pistols that are more effective against armor, and "shock grenades" that prevent opponents from firing on them in overwatch and penalizing their hit rolls. Their helms are death’s heads, with their left pauldrons enlarged to provide better protection in melee.

Their armor is stripped down to be eerily silent, adapted for stealth work (and they watch veeery carefully for twigs). Reivers are a specialized infiltration and close-combat unit. Essentially, they are Primaris Reasonable Marines. They are also totally not Night Lords, even though they wear skull helmets, are used to "terrorize" enemy units, and have a bad habit of leaving enemies flayed and out in the open—this is represented by reducing opponents' Ld by 1 simply by being 3" away, stacking up to Ld-3 if multiple units are in range.

And another interesting tidbit. The skull helmets apparently incorporate weaponized loudspeakers (Possibly 'graciously gifted' by the Howling Banshees during Cawl's time with the Ynnari). Reivers "disrupt the hearing of the enemy with vox-amplified war cries and audio-sonic weaponry", which assumes both that combat is happening in atmosphere and that the enemy is capable of hearing/didn't bring ear protection, but what can you do.

Equipment

Reivers are all equipped with heavy bolt pistols, handheld and deadly versions of the bolt rifle, with many also wielding a long-bladed combat knife. Reivers can also stun enemy infantry with shock grenades or deploy deep behind hostile lines via grapnel launchers or grav-chutes. Reivers can swap their shock grenades with frag or krak grenades depending on the type of enemy they are facing.

On a side note the “Heavy Bolt Pistol” is in a wierd state of canonicity. The lead designer for the Primaris range claims it was only added for balances sake, and codexes/BL seem to be inconsistent on wether it exists or not. (See; Vox Cast 1, the fact the absolver exists and the fact they use a normal bolt pistol model).

For additional range, they can also swap their heavy bolt pistols and combat knife with the trusty bolt carbine which sits comfortable between the bolt rifle and the bolt pistol and comes with the snazzy little foregrip kitbashers have been adding to bolters for years

On The Tabletop

Reivers are functionally useless as unless you are playing Blood Angels, they have no way of wounding T4 or higher on anything less than a roll of 4+. Be it with their guns or AP0 Knives. You're honestly better of taking veteran intercessors as they have the same melee output but for less points, and with a better ranged weapon.

Ignore the above, 41 attacks on a charge with the grenades that block overwatch and can infiltrate ob deployment is nothing yo sneeze at.