Conditions. I like my allied detachments to clock in at under 750 points. This is because I tend to play smaller points cost games than most. I still favour way less than 3000 points for a game - 2000, or even 1750 or 1500 is more than enough for an interesting game that can be concluded rapidly which is a key driving factor and a "must" most of the time for me personally - I know there's plenty of people out there who have the same limitations. I save 3000 points for when there's hours to spare.
Secondly, I want my allied detachments to do something that perhaps the primary cannot -- and to generally be fluffy. This usually means taking advantage of the Legion rules in question or its special units. For the Emperor's Children, I interpret this to mean quickly moving units, taking advantage of the quick melee weapons, and for this one: a Sun Killers squad. I also wanted to show off something very Emperor's Children by taking at least one Phoenix Power Spear!
Army List.
HQ:
Phoenix Warden, Iron Halo and Phoenix Pattern Power Spear, Artificer Armour, Jump Pack (110 points). This HQ is to show off the power spear and to be a strong threat in game. I might add a melta bomb here too.
Elites:
Apothecary, Jump Pack, Artificer Armour (70 points). The purpose of the Apothecary is obvious, and is probably a bit rude in this allied detachment, but it fits so well. That, and the Emperor's Children do like their Apothecaries, so it maintains my fluffy direction.
Troops:
Assault Squad x11; Combat Shields, 2 Power Axes, Sergeant with Two Lightning Claws and Artificer Armour (214 points). I have to take troops, and an assault squad is the obvious selection here. Again, melta bombs to taste.
Heavy Support:
Sun Killer Squad x5, Lascannons, Novaetor with Augury Scanner (205 points). This is the unit that I could not get otherwise. Yes, I could have chosen Palatine Blades, or Terminators, but this is such a unique and strong unit that really fits with the theme of what I'm going for here: long range cover coupled with up close and personal elements.
Total points cost = 599 points.
Analysis.
This allied detachment combines two very interesting elements. First we have a "standard" Sun Killer squad which is arguably one of the best in the game for lascannon slaughter that provides the long range cover. Then we have a blob of jump pack troops backed up by an Apothecary and the HQ selection. In principle, I could turn the HQ into a Praetor to add extra flexibility, but there's not much benefit to that arguably. All three warlord traits are valid for this set up, and I can see myself using any depending on loyalty. The main blob is to tear through enemy troops formations. Whether they'd want to tackle terminators is a different decision - their primary purpose is anti-troops and potentially a HQ killing element.
Set up is typically with the Sun Killers taking a back line position of defence to fire from, whilst the rest of the detachment flies or deep strikes (or even flanks) into play as quickly as possible.
Given that I have some 150 points "spare" compared to my goal, I would consider adding a Sky Hunter squadron or Xihphon Interceptor to round the list out by incorporating another fast attack element, or making the assault squad larger still (it has 11 members largely because I had a spare resin body that I glued a jump pack to in order to model the sergeant - no real reason beyond that!). There's also room to add in a second assault squad if needed to create a primary detachment too, but that's not currently a priority for me. Perhaps a defence line for the Sun killers instead?