ALSO, if you run around in my campaign, GO AWAY. I'm probably going to throw in things I don't want y'all to have.
So, a Drill City is tunneling out of the Plane of Towers and towards Necrocarcerus. Clearly you need the PCs to fight some Morlocks.
What Morlocks Are Not
This is clearly surface-dweller propaganda. Morlocks are not stupid or primitive. They have a fucking drill city! It's a city! With a DRILL ON IT! And it's coming for you! Your city walls will not stop something that can grind through the Underworld and the Underdark. They're not going to get to a stone block wall and turn around in shame. They are going to come in and rule you and destroy your civilization and enslave you and eat you. Because you are Eloi: you are soft, you are weak. The Morlocks hate you more than they hate each other. Barely.
Morlock Hate Priests
They love to use Command on surface dwellers. They love to make you kneel in their presence. They worship machines of their manufacture, but most of all, they worship Hate. They brew up healing potions of cloned Eloi blood that can heal 2d4 + 4 hit points, but only if you make a Constitution check to keep it down. Otherwise you get half of the effects and are filled with rage.
They are the peak of the Morlock caste system. They have a direct line with the Big Rage Upstairs and are listening for orders to return to the awful surface. They are also, in many cases, complete buffoons, eager to order their peons to glory, or at least horrible death. They are crafty in all the ways of staying alive and putting other Morlocks in danger. An expedition of Morlocks on the surface will be lead by a Hate Priest who lost some political infight, and was forced to leave the protective shell of the Drill City.
Morlocks in Battle
All Morlocks can tap into the Minute of Hate. They become resistant, in 5E terms, to piercing, blunt, and slashing damage, taking half of whatever people roll. They can keep this unnatural resilience up for a minute, then have to take an hour's rest to regain their smoldering disgust for existence.
Despite this, their fighters are not eager to wade into melee - like the Hate Priests, they'd rather someone else take that roll, preferring instead to fire bows or pistols from cover. But they are adept at using cutlasses and pikes in close quarters.
Priests, Guards, and Crew
The Morlocks who are not trained as fighters or reared as acolytes become crew. And the crew is ground down over time, like the Urth beyond the city. Their quarters are stacks of tiny berths, like a Japanese coffin motel built for Iron Maiden groupies. Bedding is quite basic, and communal baths are one of the only comfortable places in the Drill City for these lower-class folk. Regardless, they largely do their duty, focusing their red-eyed gaze on the Eloi. Largely.
Drill City Thieves Guild
A Drill City contains miles of iron hallway and pipe. Inevitably Morlock engineering creates, as that angry discipline always must, strange crevices, strange pockets of unused space, off the beaten path. Some Morlocks are drawn to these places, shirking the rigorous requirements of the upper caste. They might still work, but they're goldbricking when they're not actively stealing food or components for their own purposes, building up a tiny fiefdom between the steam-reclamation manifold and the primary waste filter. They'd love nothing more than to take over, eat the priests, and then probably recapitulate the same shit system all over again. But they typically stay themselves from committing outright rebellion, if it seems the priests will put more Eloi flesh on the table.
Morlock Cuisine in the Underworld
They have frozen in blocks, Eloi, or pieces of such. They take little pieces and clone them in vats, growing turgid lumps of flesh. Thawed surface-dwellers can become henchpeople or backup party members, as long as they can carve revenge into their captors and cooks.
You've probably run DnD or similar games for longer than me; you can probably come up with better stats than I can. The Hate Priests are actual Clerics, or at least, cast Cleric spells. The thieves are thieves and will shank you. The fighters throw bottles of oil and snipe at you from behind turned-over workbenches, while their 'leader' Hate Priest sings a disturbing hymn.