In the lead up to his betrayal at Isstvan III, Guilliman sent retinues to his brother's legions in an attempt to sway them to his cause. Each Legionnaire was utterly loyal to the Warmaster and his vision of the future. These retinues were not tasked to sway the Primarchs but the legionnaires. Assigned to fight alongside their brother legionnaires they would bring the Warmaster's ideals to the Legions so that the legionnaires would bring the ideals to their Primarchs.
Any player can take the following squad in a Legion primary detachment:
Scions of Guilliman Emissary Squad
Rules: they are a Veteran Tactical squad as per the rules in Betrayal, but have a base cost of 110pts
For the Warmaster!: this unit has the Objective Secured command benefit.
Veteran Tactics: Replace this Special Rule with Stubborn
of the Warmaster: This unit is bought as part of the armies parent list
but has the Legiones Astares(Scions of Guilliman) special rule instead
of the lists normal Legiones Astartes rule. The unit is treated as an
ally for how they interact with other Legions per the Allies Matrix. This unit may be purchased even if Allies are not normally allowed.
Shadows of Isstvan III: The emissary squad must be listed as a Restricted unit in any games the are taken in.