The 4e DMGII has a section on Monster Themes that confer thematic monster groups with special synergistic powers. Both "Goblin Allies" and "Legion of Avernus" have militant-themed powers and would be worth looking at for a model.
I'd give them synergistic Tattoo powers. There might also be a Divine or Location Boon that could be altered to represent something the King's priests or mages conferred to his elite guard.
I would consider giving the elite guard the equivalent of an extra +1 on their weapons and defenses, represented as magical arms and armor that confer an additional bonus to troops that have either sworn a particular allegiance, or are within 500' of their King, or are within the grounds of the palace, etc. This way the guard can benefit from the full bonus, but players defeating them get only level-appropriate bonuses on the captured items.
I'd give them synergistic Tattoo powers. There might also be a Divine or Location Boon that could be altered to represent something the King's priests or mages conferred to his elite guard.
I would consider giving the elite guard the equivalent of an extra +1 on their weapons and defenses, represented as magical arms and armor that confer an additional bonus to troops that have either sworn a particular allegiance, or are within 500' of their King, or are within the grounds of the palace, etc. This way the guard can benefit from the full bonus, but players defeating them get only level-appropriate bonuses on the captured items.