Why keep it as normal at all?
Provide them "pets" of a sort similar to the animated weapons the dorfs get to play with. They don't mine, their magically animated pickaxes do that. Same with woodcutters. So on and so forth.
No migrants at all.
These two things greatly change the nature of the game- but that just makes it more FUN!- as it stands, the alternate embark species are too similar to dwarves to really make it feel like a different game.
Later on (after achieving various goals) they can have their trade reactions to recruit additional warlocks (research unlocks which require sacrificing souls and what have you to upgrade your tower's magical status.
For example, you start by building a Locus- a building that spawns Lesser Essence for producing low end tools and undead. All it requires is stone and a warlock, of course. I'd almost suggest making it a "multipart" building. Where there's four corners that are meant to be assembled together in order to "look right"- so it can be a larger building that is impressive looking, while being usable by more than one warlock at a time.
Using the essence, and more typical workshops, you can create refined materials for higher end buildings.
Say, building a Grand Library in order to unlock the research material.
Undead Chrysalis for invasion mechanics and spawning undead pets for your fort (anywhere from 5 to 20 used per reaction- different difficulties and possible rewards based upon what you invade- increasingly low returns for higher level invasions, but much higher possible supplies).
Slave Pits from captured slave "items".
The Transmogrifycation Chamber- a building that uses slaves, undead, and essence to produce various abominations that may or may not be used to make new buildings.
Torture chambers that grant you access to race-specific invasion buildings that offer better chances and rewards (but require a large number of captives to build).
But for a goodly couple years, they should be limited solely to their embark units. You'd basically end up controlling them by assigning them as leaders of military units, and most of the minions as following pets.
Slave collection would be the bulk of their long term industry. "Slave driver" as a skill that most of their workshops exploit. Perhaps break it up by species. Human slaving, Dwarf slaving, Goblin Slaving and so on.
Undead minions would mostly exist to do hauling labors and kill invaders. Start with basic corpses and have an increasing upgrade of horrors to unleash.
See if it's not possible to get different corpses based on mode of death. Would be interesting to see a drowned corpse or a charred corpse to turn into different breeds of zombie.
I can see industries as follows:
1- Locus- Centerpoint of warlock life. Spawns Lesser Essence. Which is used in almost every lower reaction Warlocks perform.
2- Crafting Building- used for low end stone and wood reactions (plus needs essence). Warlocks can have a distinctly lacking number of reactions- because later they'll have the good stuff from slaves. Produces "glamored" items.
3- Enchanting Room- uses various ranks of essence to generate various grade magic items and/or pets.
4- Echo Chamber- low end magical training room. Eats essence, produces the vast majority of early magical advancement.
5- The War Room- which spawns items for various degrees of raiding (a-la the expedition outfitter) and its sister building(s). Options such as "Invade (race) outlands"- low risk, low reward, offers very good chance of coming back with 'meat' supplies like pets and children or peasants of a race. "Invade (race) hamlet" for more moderate risk-reward. Town. City. Fortress. Capital.
6- And the necromancy chamber. Researching the dark arts and spawning off lesser undead. Except to feed it lots of corpses.
Then you get medium tier stuff.
1- Heartstone. THIS lets you start attempting to recruit "potentials". Possible evil wizards to join your army. Also more commonly used for Greater Essence (cornerstone of higher end production). Again with the 'four corner' idea.
2- The Black Spiral (hehe, WoD reference, feel free to change the name)- a BIG building which requires bones, corpses, essence, and a handful of crysalis- the potentials will be fed into this reaction. If all goes well, a new Warlock shall be added to the fold. Far more likely, you either get a Crazed Wizard that you must kill. Or a corpse. Or an Abomination Soul (for high end crafting).
3- Torture Rack- either high end T1, or low T2- this baby is for interrogating prisoners. Reactions include-
A: Interrogate. Has a number of results- including civilization notes, getting the prisoner back with no results, getting a dead prisoner (with or without notes), getting a prisoner revolt- and a fight on your hands, have guards nearby- or getting a useless prisoner and notes.
B: Enslave. Favored reaction for soldier enemies. Spawns you a slave to do your dirty work. Generally not the best way to go, but they can always be butchered in the future if you need the corpse for another project. Waste not, want not. Slave is based upon type of slave used, and a lot of luck.
C: Convert. A specific and rare reaction that ONLY occurs with nobles. Gives your warlocks dastardly things to do like forcing beautiful princesses to marry them. Always a classic.
4- (Race) War rooms. As said on the tin. Lower odds of minion loss. Somewhat better item rewards. Also unlocks the possibility of finding "potentials" during an invasion, and bringing them home with you (see the Spiral and its possibilities)
5- (Race) Slave (Workshop)s- dwarven smithies. Kobold crafters. Human farming. List goes on and on.
6- Start of the magic system "for realsies". Animated tools (lesser animating using Essence- the greater kind... greater animating using souls). Soul harvesting from the necromancer chain. Possibly acquiring the weaker demon familiars (imps and homonculi) who ever so kindly cast lesser spells and offer temp stat boosts to their allies.
High Tier materials
1- Summoner's Chamber- THIS is proof that your wizard tower is now respected amongst the evil community. A big room, this time only one. Used for demon magic(s). Offering souls as reactions. Recruiting warlocks without the possible risks- though a higher price.
2- Fully upgraded torture mechanic- requiring a huge number of sacrificed captives to potentially acquire, you get the "Detailed Notes" on a given target (and each civ has those aforementioned notes)... letting you take those notes and an item from the prior chamber and generate an even superior set of invasion options.
Adds the "target trade route" option. Which guarantees lots of material loot and better-than-average captives.
If you have extra notes of a specific target around, you can initiate the "decisive blow" command. A high power invasion which expends dozens of minions and guarantees the triggering of an invasion- potentially multiple invasions simultaneously- but results in huge amounts of loot.
3- Fully unlocked magic systems. Necromancy. Demon Summoning. Infusions to create demonic hybrids.
Slaves-
You can acquire different types of prisoners from different locations, but there's many potential types. These are *items* to be consumed in other reactions. Each race has their own equivalents.
1- Child- most common in the outland raiding, progressively less likely as you get more urban. Safest, but least valuable, reaction. Used to spawn ankle-biter zombies and skeletons. Also possibly butcher for small amounts of meat and bone.
2- Peasant- found everywhere, though mostly human and dwarf. Good for base tier undead, butchury, and etc.
3- Sniveling peasant- found amongst kobolds and goblins and some humans and dwarves. THESE slaves are less likely to fight back when undergoing reactions where they don't die. They also mostly suck and are rather a waste to keep around for long.
4- Warriors- soldiers of varying descriptions- found amongst every race. Great potential rewards in terms of somewhat superior undead and minions. High risk of fighting back when used to generate servants, but best reward in terms of minion strength on success.
5- Tradesmen- necessary to produce most of the slave workshops.
6- Mystics- The wizards and shaman of enemy races. Dangerous in any reaction. Easily sold to demons for high value rewards. IF you convert them, they become warlocks.
7- Traitor- A rare find. Automatically gives you "Notes" and "Useless Traitor" when interrogated. THEN can be used in other reactions- either feed it in as a Potential or a Servant, depending upon luck from the reaction (as they're traitors, their interrogation is a lot less painful than normal, so they aren't left broken afterward. ALSO a favored sacrifice to the demons. Seriously. Traitors are like Cocaine to them.
8- Powerful Warrior- Different classes based on race, but all in all, the best you can get for bolstering your armies. Make the best slaves (and thus some of the best undeadification picks). Makes the best sacrifices. AND makes the best interrogations. Also, in all cases, the best odds of prison breaks.
9- Royalty. ONLY possible from a 'decisive blow'. Ranging from mayors to kings. Includes fun things like trying to convert a princess into a Dark Queen, or a prince into a Black Knight. 'Ransom' reactions.
10- "Useless prisoner"- a possible result from the Interrogation reaction(s). These prisoners can't be used to interrogate any longer. Also- if used in other reactions, they produce inferior results (identical to the "sniveling peasant"- which makes them perfect for that reaction. Best used as food. Also a fairly likely result from raiding.