My thoughts on viable factions within the grand allegiances that work and which ones you need to use multiple factions. First off, some terms for new players that will appear here: Battleline, battleline is a unit type, sort of like a Troops choice or Core from other GW games. What that means is that there's a requirment to bring a certain number of Battleline units for your list to be 'legal'. The number of Battleline units required depends on the total points value agreed upon to play. So for example in a 1000 pts battle you need to bring 2 units who are battleline. Which units are battleline are explained in the generals handbook (or on the Age of sigmar app).
The second term that can be a bit confusing is Allegiances. What these generally mean though, is that if you bring an army where all of the units share a keyword that belongs to a faction, like Deathrattle for example, your army has the Deathrattle allegiance and may gain certain benefits from this. Sometimes you can pick command traits and artefacts only avaliable to that allegiance, and other times it might make a certain unit count as being a Battleline unit. Now not all armies have a battletome, as you will see below only one faction has one. But fret not. If all your units are from the same Grand Alliance, like Death, Chaos or Order. You get access to some general traits and artefacts (and a battle trait), meaning you're usually much better of sticking to one of these.
Death:
Flesh eater court: First of the death factions to get a Battletome, lots of synergy in-faction and some stuff in Death can buff them a bit, no Flesh eater specific relics/command traits so no reason not to splice in from other Death factions though. Their Battleline (Crypt Ghouls) isn't dependant on being Flesh eater allegiance either so that's no restriction either for non-flesh eater lists, but they perform a hell of a lot better within their faction with buffs and characters refilling their ranks. Overall their theme is ambush and leaders that either bring on new units or replenish existing ones. Beware the Terrorgheist's bite! Now you know why you fear the night.
Deadwalkers: Just 3 units, so not viable as a single faction but instead there if you like the zombie theme over ghouls or skeletons. Zombies are battleline in Death too so you can bring them in a general Death allegiance army as your required battleline troops.
Deathlords: Mostly the Mortarch heroes and Nagash, plus some powerful units in the form of Morghasts who are battleline in a Deathlords allegiance army (aka only deathlords). Doesn't sound that viable as it's 240 for two Morgasts and then between 340-900 pts for the Leaders who also are behemoths. No, you take these to bolster a Death Allegiance army, the Mortarchs make good generals, and a unit of Morgasts are pretty darn killy.
Deathmages: Consists of Necromancers and the Mortis Engine. 100% just something to add to a Death army, not something to pick as its "Core theme".
Deathrattle: This can easlily form the bulk of a Death army if you like the skeleton theme, skeleton warriors are straight up battleline, Black knights are battleline in a deathrattle allegiance. Other than that though they really only got 3 versions of a Wight king and some semi-heavy infantry called Grave guard. Their central theme is that they get models back in your hero phase. More of a tar-pit army, they will need some heavy hitters to support them.
Nighthaunt: Almost like deathrattle in that they could form your base or be your main 'theme'. On the other hand, only two units with their (battleline if Nighthaunt) Spirit hosts and cavalry hexwraiths makes them more likely to be filler. Like with Deathrattle a small army could easily be all nighthaunt but once you go up in pts you're going to be pretty limited, lack versatility and more hard hitting units etc. They have three characters, the cheap and quite killy against swarms Cairnwraith, the Tomb Banshee who has an attack against bravery and the "old-hammer" black coach that powers up from nearby death wizards (so requiring you to bring Deathmages, Deathlords, Flesh eaters or Soulblight into the mix). Nighthaunts specaility is lots of flying and immunity to rend, both are pretty good abilities, though they will melt to mortal wounds despite being ghosts.
Soulblight: Vamps! If you are dead set on making a Soulblight allegiance army you're going to have to be very limited as the fearsome and expensive blood knights are your only battleline, eating away at your points. I feel it's better to use skeleton warriors or ghouls as battleline, and spice up a Death army with Soulblight instead. They do have quite a few units so if they ever got another battleline they'd be quite good to go as an army in and of itself (fingers crossed). They have solid leaders with mostly buffs to Death keyword like the vampire lord, unlike other heroes who buff specific keywords like the Wight King for example. The vamp lord can also come on a zombie dragon, which can hit like a brick and tank like a god if the lord has a shield. 3+ save, abilities to regen wounds? Yes please. Other than this there's the Coven throne that has some shenanigans, fell bats and bat swarms (bat swarms debuff enemy ranged units in an AoE) and lastly Vargheists who can hit pretty hard and fly about but aren't the toughest guys around.
Verdict: If you don't play Flesh eaters you will probably splice in a little from every faction for Death, until they (finally) get some new love from GW. Deathrattle, Nighthaunt and maybe Soulblight are the beefier picks as a core for your army, but you should probably sneak a peak at some Deathlord models too and maybe take something from several of the factions. For example a Necromancer can buff basically anyone with the skeleton, mordant or zombie keyword with his awesome Dance macabre spell (makes them pile in and attack a second time in one go), for some beefed up damage output, just make sure to keep him safe, So he could easily be added to a Deathrattle themed army to add some teeth to your skellies.
