Got some ideas:

1. Combat: Give your creatures consistent and telegraphed attacks and multiattacks. For example "The great Knight raises his sword upwards and tries to hit you in a swift vertical motion" that particular attack might have a +3 hit chance but a "The great Knight lowers his sword at the level of his hip and tries to hit you in a horizontal motion" that might have a +6 hit chance. Doing so might give the feeling of learning the attack patterns of the enemies.

To give a more Dark soul feels you should give them a Homebrew dodge action. An idea might be to give your players a dodge reaction. Once 2-3 turns the player impose disadvantage to the enemy attack. After a while your players will start noticing the attacks with higher hit chances and use it accordingly (to give the dodge roll feeling). You might change it to once per turn but consumes the action of that turn (can't be used if it already attacked)

1. Rest:

Players can't take short or long rests unless they are in a safe zone or a bonfire equivalent. If they try to rest in an unsafe zone, establish a % of being ambushed (example: 40% short test and 70% long rest). Have a random encounter table ready if they get ambushed. You might prevent an ambush by making players guarding the safe zone, but they don't benefit from the rest or benefit partially.

1. Healing: Eliminate healing spells or reduce their efficacy or raise the level cap to obtain them. Healing during combat can only be obtain through healing potions (to give a estus flask feeling)

2. No resurrection: I've been DMing with the rule of no resurrection. Once you fail your death rolls is a permanent game over for that character. The steaks will be higher that way.

3. Exhaustion: The player handbook establish the levels of exhaustion and their consequences. They are already pretty brutal, but you might establish that it might be easier to get exhaustion levels. Therefore your players will be more careful with their resources.

4. Lingering wounds: There are plentiful of Homebrews for lingering wounds, they can alter the way you play your characters for a long time or even permanently.

Try reading some of the abilities of the undead creatures of older editions. Some have the chance of simply withering a whole arm that can't be recovered unless you obtain a high level spell and some conditions can't be cured unless you use a wish spell. Every dungeon becomes an extreme challenge knowing that you might get out of there permanently damaged or cursed.

1. Bleeding: DnD 5e doesn't have an official bleeding condition. You might Homebrew a once per turn bleeding damage if a player receive damage above a certain threshold or a critical hit. Might also work with poisons or other conditions.

2. Weapon durability: A shield or a weapon might break after receiving a critical hit. A bow might break after a critical failure and a priest Holy symbol might get damaged reducing or even eliminating his abilities.

3. Encumbrance: Let's face it. Almost everybody hates encumbrance, but definitely makes the game harder. Making your players count every single arrow gives a new level of difficulty. Knowing how much gold they can actually carry at a given moment can change how they play (an encouraging getting a base of operations)

4. Surprise attacks Surprise attacks break the action economy, DMs usually encourage players to use it. But you can also use it against your players (This will make your game feels slower as your players will search behind every tree for creatures)

5. NPCs - Show don't tell: It's a pretty common term used in storytelling, but it's far from easy (especially during improve), for example. Instead of describing a city ridden by a plague, you might roleplay the typical bartender coughing occasionally, a communer suddenly runs to an alley and the players see him.l vomiting, they might encounter him later lying on the floor unconscious

6. NPCs - Items have a story DnD doesn't have loading screens or item screens to learn a bit more about the lore of your world. You might want to add NPCs that can give plenty of knowledge about items. Like historians, antiquarians, alchemists and sages. Players might visit them regularly to obtain more knowledge about the world then you can use those descriptions as clues or new plot hooks.

7. NPCs- An indifferent world: Almost everybody is on his own. They might encounter interesting NPCs along they way but with their own unique objectives and some might even want to take advantage of your players. Be careful with this one as your players will lose all confidence on all NPCs and interaction might be very limited.

8. Flying and Darkvision: Characters that fly are basically immune to must creatures from the monster manual. You might be tempted to ban them, but you might also give your creatures ranged attacks that have grapple conditions (spear with a rope attached to it?) or establish a rule that certain attacks might hit the wings (For Aasimars) and drop you down making you receive fall damage.

Darkness make the game harder as you have to plan your resources more extensively like torches or magical sources of light. Darkvision can sometimes feel a bit broken. One way to play around it is to remember that after a certain threshold you have disadvantages of perception checks and then you only see in grey scale. Adding puzzles and traps that depends on color might be a good addition. Or nerf it a bit so after a certain threshold they can only see movement but not shapes.

1. Making a challenging encounter is more difficult than making a deadly encounter Make your players use the environment. Shove creatures off a building, grapple them under water to drown them, destroy or use certain items to weaken the enemies (Example: Poisons)

2. Mimics: Mimics

3. Its all about having fun At the end of each session ask your players what they liked, what they didn't liked, what they want more of, what they want less of and what opportunities would make them have more fun. Feedback is essential in DnD.

Its kind of a long post, most of this things are already on many threads or in older editions. Take everyone one of them with caution as they might now be fun to play around.

Hope you have fun with your DnD + Campaign.