Besides sympathizing, which I do, believe me, I don't feel my post was in any way helpful. It's honest, but not helpful.
So here's a (hopefully) helpful post. Try some of these...
Environmental Hazards. Use environmental obstacles (treacherous terrain, extreme weather conditions, or collapsing structures) to add interest and spice to encounters. Mud, flowing water, poisonous mushrooms, anything. Research some roughly corresponding real-world environments to get the imagination going. Collapsing bridges over lava flows is a personal favorite. Extra points if the lava is infested with some kind of lava-crocodiles. "Hang on, lady. We going for a ride!"
Tactical Encounters. Design encounters around strategic positioning, cover usage, and that require teamwork to overcome, rather than relying solely on brute force. Borrow from 4E for monster and encounter design. If you don't have it and can afford it, check out Matt Colville's monster book.
Consumable Management. Push scarcity of resources like healing potions, scrolls, and/or ammunition. Force the players to ration their supplies. Of these, the players will likely rebel against this one the hardest. If they're in the Underdark they're not in friendly territory. They might be able to forage food, but they can't make their own healing pots, scrolls, arrows, etc. So just stop rewarding them.
Action Timers. Use time-sensitive challenges or objectives, put pressure on players to make quick decisions and act decisively before time runs out. This can either be a sand timer at the table for turns or in-game timers like the ritual will complete in 1d6 rounds, which means the Dark God of Wobbly Prunes steps through and starts murdering people.
Customized Monsters. Modify existing monsters or create new ones with unique abilities, resistances, or weaknesses tailored to exploit the party's vulnerabilities. It's kinda cheesy, but considering you're dealing with a gloomstalker, cheese is definitely on the menu. Something like Forge of Foes or 5E MM on a Business Card from Blog of Holding are great resources.
Dynamic Encounters. Create encounters that evolve over time, with reinforcements arriving, environmental conditions changing, or new threats emerging as the battle progresses. Basically, combine the other encounter advice to make a giant convoluted brawl of a fight that will, hopefully, push everyone to their limits.
Non-Combat Challenges. Use puzzles, traps, social encounters, and skill-based challenges that require creativity, problem-solving, and role-playing skills to overcome. The 4E skill challenges were terribly written, but the general idea is sound. Check out PBTA games and BitD games for their use of clocks. Start tracking lots of non-combat stuff with clocks and use them to drop stuff on your PCs' heads.
Dynamic Game Difficulty Balancing. You see this all the time in video games. Adjust encounter difficulty on the fly based on the party's performance, ensuring that encounters remain challenging without becoming overwhelming or trivial. This is a bit controversial. If you explain things in the fiction with obvious and visible power ups, it's probably fine. As long as the players can maybe prevent it, go for it. They can't miss, their target disengages, grabs a +2 shield and downs a fist-full of healing pots before charging back into the fray.
Specialized Tactics. Make baddies that employ specialized tactics, formations, or synergies to counter the party's strengths and exploit their weaknesses. You're in country. You're in enemy territory. You're in the Underdark. There are going to be special forces types that catch wind of the party and start hunting them. The Underdark version of Fantasy Vietnam. Tucker's Kobolds. Only nastier.
Limit Rests. Restriction rests. Force players to find a safe haven to get even a short rest. If you've already got someone with Leomund's Tiny Hut, use the various exploits others have posted. If not, ban the spell. Use something like Safe Haven resting rules. This is another that will go over like a lead balloon. (Yes, I saw the Mythbusters episode.)
Morale and Surrender. Use the old morale mechanics for enemies. Have them flee, surrender, or negotiate under certain conditions. Not everyone fights to the death. Have the back line flee mid combat to alert others nearby. This can result in reinforcements or areas that the locals have already cleared out so they're not slaughtered by the party. Might end up with prisoners, too. Watch out for that. Anyone serving a Good deity? That's not going to be a great conversation to have.
Political Intrigue. Use political machinations, faction play, and power struggles in the campaign, challenging players to navigate complex social dynamics and make difficult diplomatic decisions. Focusing on the politics of the Underdark can be great. It can also be a nightmare. Which is perfect. It makes things more complex that just hack-and-slash your way through the rooms.
Cursed Items and Artifacts. Give the PCs some cursed items or powerful artifacts with dangerous side effects or hidden drawbacks, tempting them with their benefits while posing significant risks. Another controversial one. But it works a treat. Just remember that people view negative things more starkly than positive things. So it's got to be a great benefit and a middling negative at worst or they won't use it.
Economic Challenges. Create economic challenges such as inflation, scarcity of goods, or economic downturns, requiring players to manage their finances and resources. Especially if they're in enemy territory most of the time. If no one's trading or selling them stuff, it'll be hard to survive. But also, they're bringing surface coin, the locals might not accept that. If you're into the extra paperwork this can help with immersion a lot. It can also be tedious as hell.
Moral Dilemmas. Present players with morally ambiguous situations, ethical quandaries, and difficult choices that challenge their convictions and force them to confront the consequences of their actions. If they're just murdering their way from one end of the Underdark to the other, they'll not likely face any lasting consequences...unless some of those consequences come find the PCs. Those special tactics and special forces from up thread. "Hello! My name is Drizzleplix! You killed my father! Prepare to die!" They're looking for two NPCs. When they finally find them, they're being held over a chasm by a madman. One from each arm and he's threatening to drop them both. The PCs have to decide what to do.
Make Complex Baddies. Develop multifaceted antagonists with complex motivations, sympathetic qualities, or redeeming traits, blurring the lines between hero and villain. This ties back into the moral dilemmas and politics. Don't make things black and white. Paint with shades of grey and don't let simply stabbing be enough to always win. Put baddies in unreachable places, behind impenetrable barriers. Go all pulp and comic book melodrama with it.
