0. Do your players want this, too?
If you haven't discussed a different playstyle I think they're reasonable to still expect the "sporty" style you were previously playing. That's why I think--even one session in--a change like this would be equivalent to starting a new campaign: same setting and same characters, but different game.
If you have not had that conversation, stop reading here. Talk to your players. If they agree on a new style, proceed:
1. Do it to them. Before you do it to them, tell them you're going to do it to them. And tell them while you're doing it.
"The hobgoblin captain has arrayed his shortbowmen on inaccessible ground and they're ducking behind full cover after each shot. Man, this is just a killing field! So, Gary, what do you do next?"
"OBJECTIVES, TERRAIN, COVER, VISIBILITY" are the four words I have written on my gm-facing side of my table tent. Use these to your advantage, and teach the players to use them to theirs.
2. Design your world, not your encounters.
Think about the people in your world, where they might exist, and what they value. Array them as makes sense in a world without your PCs. Do the encounter math as suggested by the DMG. Then don't modify anything!
Now you have a range of possible encounters including (a) not worth even acting out--just declare victory, (b) cake-walk, (c) very easy, (d) easy, (e) medium, (f) hard, (g) deadly, (h) superdeadly (would be deadly even at APL+1), (i) superduperdeadly (deadly at APL+2), (j) death sentence (could kill the party before they even act).
I'm not kidding: "no," "cake," "VE," "E," "M," "H," "D," "D+1," "D+2," and a frowny-face with two exes for eyes are notations next to possible encounters on the mind-map of my current adventure.
Now that you've got this in hand, you are prepared to properly describe the encounters as they happen. In a "cake" encounter your barbarian's axe cleaves enemies in twain (doing 9 dmg); the same hit in a D+1 encounter manages to annoy the enemy. But if you didn't do the homework, you're going to properly convey to the players the crucial information their characters would know: "we're in over our heads, here, and will be lucky to get out alive."
3. Help them find their way to spectacular victories.
There are many gm-styles and I'd naver say one is 'right' or 'wrong.' But if you're looking to help a group transition their style--or to learn a new style--I suggest that the one I call "find a way to 'yes'" may be really useful. In this approach you are explicit about asking players not only to describe their actions, but also their intentions. You work with them to craft their actions so that they to progress toward their objectives.
You don't have to pull any punches in combat or in their opponents' preparations and they don't always have to succeed. But this way their efforts aren't impotent even when unsuccessful. Failures aren't a matter of "the gm screwed us over," but are a matter of "the kobolds screwed us over." And successes can be spectacular. Like Hannibal says: "I love it when a plan comes together."
This is quite a hard question, so I will do my best to summarize what I've found on Orcus and Demogorgon. Also, the 4E Book of Vile Darkness has a lot of information about demons lords.
Orcus
- Lieutenants: Vampires, liches, necromancers, cult priests
- Minions: Undead (ghouls, shadows), cultists
- Races: Not relevant for undeads, humans, orcs, half-orcs, ogres and giants
This may be the easiest one, because of all the detailed description about him. From the MM p. 52:
Orcus is worshiped by the undead and by living creatures that channel the power of undeath.
Which makes the choice for a lieutenant relatively easy. On multiple wikis (like here or here), liches and vampires are mentioned as his most powerful worshipers. In the 4E published adventure Keep on the Shadowfell, a human priest of the Cult of Orcus tries to open a portal to the Shadowfell. Also, from the MM p. 53:
Orcus, the Prince of Undeath, has the power to transform manes into undead monsters, most often ghouls and shadows.
We can assume he does that to make them serve him as minions. From the Book of Vile Darkness p. 138:
In particular, orcs, half-orcs, ogres and giants revere Orcus, as do a large number of corrupt and despicable humans.
Another quote from the BoVD p. 138:
Intelligent undead never willingly serve Orcus (they are more likely to venerate the deities Vecna or Erythnul). However, many vampires, liches, and other undead creatures are forced into his service by dark pacts or compelling magic.
Demogorgon
- Lieutenants: Severik the balor (mentioned in the BoVD p. 129)
- Minions: hezrous, aboleths, scrags, skum, various ichytoid beasts (BoVD p. 125)
- Races: tiefling, lizardfolk, half-elf (examples from the BoVD), kuo-toa (Out of the Abyss), Derro (Out of the Abyss). Mostly any kind of creature that has reached a pretty high level of madness.
Not many quotes for the Prince of Demons, but you will find more details in the BoVD.
Other demon lords
Well, when I started writing this answer, I thought I could gather information from websites, books and the like, and summarize my findings here. It turns out, the Book of Vile Darkness from 4E is really the best source of information about the demon lords, so you should probably buy it for the other ones.
Best Answer
In the Dungeon Master's Guide (pp. 267-268), there are rules for firearms, including future items (e.g., laser pistol, antimatter rifle and a laser rifle, along with modern weapons). Also explosives, including grenades. There are also rules on 'alien technology' in the DMG (pg 268) which is rules for a character figuring out how to use technology works.
For most technology (magi-tech or otherwise), I would use these rules and simply treat it as a magic item with similar properties. Jet pack is a broom of flying, a computer is a scroll of some divination spell (with recharging charges) and a hovercraft is an airship or flying carpet, etc., perhaps with limited charges.
Since the DMG also features guidelines on making spells, you can make up an ability with those guidelines and treat items as either scrolls or wands to simulate technology.