dsfarwell wrote:My son is using a Culexus Assassin as an ally detachment with his Ultramarine chapter. However, this assassin is only one character. To have a bounded army using an allied detachment page 122 of the latest version of 'The Rules' indicates you need an
HQ and Troop choice. What/where is the rule that this one character can substitute for this requirement? dsf
Have a look in the Officio Assassinorum Dataslate. There is an additional Detachment you can use which consists of a single Elites choice chosen from Dataslate: Officio Assassinorum (in other words, one of the four Assassins).
There are TONS of additional detachments in Codexes, Dataslates, Campaign Books, White Dwarf, etc. The core rulebook only shows the initial two which correspond roughly with the primary and allied force org charts from 6th edition.