Thanks to Spanish 40k site La Voz De Horus we can now see what the Reivers are all about:
Thanks to the magical wonder that is Google Translate, I can tell you what they do!
The first weapon is a bolt pistol with AP -1 (pretty simple to figure out)
The Filoarma (combat knife) provides an extra attack in close combat
The next two are frag and krak grenades, you should know how these work.
The last one, the Shock Grenades, are the new cool thing on the block. They do not inflict damage. Instead, each time it hits an enemy unit, it is stunned until the end of the turn, it can not do overwatch and your opponent must subtract 1 to any hits made by the unit. The shock grenade has no additional effects.
That is pretty cool! As we have seen, the Reivers will come in units of 5 to 10 and now they appear to be really good at getting into close combat. What do you think? Will you be picking some up?