As a pet-class lover myself, welcome to City of Heroes!
Demons/Dark is one of the strongest pairings in the game. I have one at Incarnate level and its one of my top characters both for visuals, fun factor, and taking on the game content.
I don't do any builds any more in Mids character builder, but if you search this subforum you can find some posted builds for this combo.
get all three pet powers, Hell on Earth, both pet buffs, and choose one or more of the attacks (I take the ranged whip power, which helps me either pull or help debuff damage resistance).
Six slot the Hell on Earth when you can with all six of the Mastermind Invention Origin (IOs) enhancements that provide a buff aura that boosts Damage Resistance and Defense of your pets. This is essential for making pets tough enough to handle content at higher levels/purple conning. Start with the Defense ones first, then add the Resistance ones, depending on price. Its a damage buff power that also spawns some fragile minions, but its actually best used to mule the pet IOs so they don't need to go into your Demon power slots.
Slot the Demons with accuracy, damage and some END reduction. For basic IOs, 2 Accuracy, 3 damage, 1 Endurance reduction. Don't both with recharge. For more advanced slotting with invention origin enhancements, see some of the other guides). Demon Prince especially likes to suck down his blue bar (endurance) so he benefits from some advanced slotting. He can also take a damage proc in his lineup as most of his powers have a slow component than can trigger the damage (again see advanced guides).
the two buff powers are fine with one Endurance recuction each.
You can delay the Whip attacks until you have time, as you have quite a few powers to take up through your twenties/thirties. When you do, basic slotting is Accuracy, Damage x3, endurance, recharge.
you want everything except Black Hole (which is not in synch with how the game actually plays). Petryfiying Gaze (your single target hold) is not mandatory, but can be handy to take out annoying lietenants (such as Malta Sappers), or stack with your Dark Servant/Demon Prince's own holds (Demon Prince especially as he spams his ice hold), letting you take down Bosses. Don't pass up on Howling Twilight, as although its a ressurection power, it actually comes with huge debuffs and an AOE stun for control. In fact, most powers in Dark Miasma serve multiple purposes.
These will be your meat and potatoes powers: Twilight Grasp. Tar Patch, Darkest Night, Fearsome Stare, Shadowfall.
Lead off with Fearsome Stare, its a big cone and it not only makes targets afraid, but also debuffs their To Hit/accuracy. Or you can lead off with your toggle debuff, Darkest Night.
Slot Darkest Night with -To Hit and some END reduction.
Slot Fearsome Stare with -To Hit, not fear, as the fear lasts long enough, and its the stacking -ToHit that you want.
Dark Servant is a handy pet. Slot him (or her) also with -ToHit, as he has absurd levels of it, especially in its point blank debuff aura Chill of the Night. His heals, holds, damage are all secondary and work well enough without any focused enhancements.
While Demons are tough, and Shadowfall and pet aura IOs add greatly to that, Dark Miasma really does't buff, so you must be proactive and maintain your debuffs on your foes. Dark Servant will help when you get it, though its supplemental, as you can't control it. Dark requires a bit of concentration, unlike say Force Field, where you can fall asleep at the wheel.
Leadership - at the very least, take Maneuvers and Tactics. if you have room you can take Assault. I generally don't.
Speed - I take Hasten on all my builds, as having powers recharge faster is too good to pass up.
Flight - I take Hover, Fly, Group Fly and possibly Afterburner. You can take what fits your theme. I like Group Fly because while it may annoy some teammates, it helps my pets on the rare occasion Z axis maps cause pet pathing AI to go wonky, running along beams or jumping up and down off ledges, etc.
Some folks really enjoy the keybind commands for masterminds, but I get buy admirably with just my basic GoTo/Stay and Aggressive/Defensive/Passive commands 99% of the time. How intricate you want to get with controls and emotes and all that extra fun stuff is up to you 🙂
I can't stress this enough, do NOT speed level. City of Heroes/Villains has some great missions and story line. (yes, the earlier blue side stuff, can include some clunkers or repetative missions, but overall...its really good). For your first time playing through either blue or red side, stick with the story missions. Read through dialog. Look at and read your clues in the clue window. There's so much good stuff, background history and references, it would be a shame to miss it by jumping into a farm group or speed radio XP team. Take your time, enjoy. This game has some deep lore in it, some of the best original stuff I've come across in MMOs.
I may have missed a few things, but you get the general idea.
Have fun. If you are on Everlasting, feel free to grab me at my global (@Force Redux) and I can jump on my Demon/Dark and do some missions with you. I may even have a few IOs I can help you with. I don't level many new toons these days and I'm happy to do story missions, etc.