The Victrix Guard are in all essence the spiritual successors of the Invictus Guard from the Great Crusade. They are an origination created by Bobby G to be the body guard and envoys of the Primarch and other Ultramarine officials. Considering it was Mr Politician that created them, the Vitrix Guard act as politicians as much as they do warriors, and they regularly study diplomacy and political tactics to help further enforce their Primarchs will and orders to the Imperium.
The Victrix Guard are lead by Cato Sicarius after he lost his job of the Second Company when he was missing in the Warp for a few decades, and are made up of a combination of Ultramarine Veterans and Honour Guard units. A contingent accompanied Guilliman to Terra, while another contingent was sent to Vigilus with Marneus Calgar to aid in the defense of the planet, however most of these where made up of Primaris Veterans, with "bitchin" shields and helmets to boot.
On a side note "Victrix" is latin for Victress (A female victor/champion), meaning the Victrix Guard translates into "Victress Guard" or "Guard of Champion women".
Ironic considering all Space marines are male, and once again showing that while GW "knows" history they seldom take the time to fully research it. Historically, all the Roman legions were named with feminine names such as Ferrata or Fulminata, likewise all continents were given feminine names as well (Europa, Africa, Asia...) by the Romans; the two last faiths of the Romans before full conversion to Christianity were the solar cult of Sol Invictus, the Unconquered Sun, rising from the realm of night and bringing back activity and light in the world of men, and th cult of Victoria, latin name for the greek goddess Nike, who survived even unto Christianity, and influenced along with Hermes the antropomorphism of angels (who looked weirder in pure Judaism): in the Imperium this is paralleled by the winged Living Saints of the Ecclesiarchy. Hence, the "Victress" of this special corps might simply be a dedication to a blend of Astartes cult (goddess both of war and love, as was Aphrodite in Sparta) and mainstream imperial religion. Hence, this time, G.W. didn't actually messed up (though whether they gave them stats and rules reflecting the Victrix Guard's lore is another story).
On the Tabletop
The Victrix Honour Guard have a fairly decent stat line, being essentially Honour guard with an extra wound, 4 attacks and with the Bolter and Power Axe replaced with a Storm Shield and Power Sword. They come in groups of two, and while the lack of a ranged option makes them a bit inflexible and the Power Sword has slightly limited their killing potential, the Storm Shield more than makes up for this, as any charging character killer will have to punch through a total of six wounds with a 3++ invuln, before getting to your Warlord. They can also make Heroic interventions meaning you can actually cover a fair bit of an area with a unit of these guys and protect multiple Characters with them.
There is also a Specialist detachment in Vigilus Defiant, to give all Captain, Honour Guard and Veteran Squad units in a detachment the VICTRIX GUARD key word, which gives them access to unique stratagems, relics and warlord traits.