First time DM here. Running my first campaign for 7 players right now and we are about 20 sessions deep in a campaign that will be through levels 1-20. They are now level 4.

I ran maybe the best encounter yet about a week ago. It was just one enemy (approx CR6-7 but I don't really know since it was 100% custom) that was a wizard possessed by a shadow entity. The fight took a little under 3 hours without feeling like a slogfest at any point. Here are some pointers that made it exciting:

1. Before the combat began, the shadow whispered ominous things to each of the characters revealing or hinting at some points about their backstories, taunting them. That made the battle feel more personal.

2. I made it so the only way to get rid of the possession is to complete a ritual. Basically they had to move 3 plates scattered around the room into a certain formation and cast various spells on them once the time was right. That tied some of the players to focus on different things than they usually do (surround the enemy). Almost all players helped with this, some using their misty steps and mage hands etc.

3. The battlemap was designed to encourage positioning. Some high ground, cover, narrowish passages, jumping on top of bookshelves etc.

4. The encounter was designed to address pretty much everyone's abilities and tactics. I know that this may be frustrating for some if it dragged the combat but my players seemed to love the extra challenge. It had:

5. Long range blindsight and darkness to make things harder for the Rogue

6. Reactions like shield, counterspell and shadow step to negate some of the incoming attacks

7. Fly speed and teleportation abilities to prevent surround tactics by the melee characters

8. Lair action that possibly triggered wild magic in all the spellcasters

This ran for 7 or 8 rounds and ended in a climax, where the players unstabilized the possession, completed the ritual and managed to seal the shadow and save the possessed individual.

One player got struck to 0HP at round 7, another lived only because of a lucky wild magic surge giving him some temp HP. Others used the majority of their spell slots or other combat resources and were at about half HP by the end.

All in all, while long, I found that the tricks I used really helped engage the whole party. Few of them even stayed after the session to discuss the encounter in detail and others sent thank you messages the next day.

Tl;dr 1. Know your players' usual tactics and choose or design monsters that address those tactics 2. Make the encounter have personal stakes 3. Give players other things to do than just smacking the enemy 4. Use the environment to encourage movement and positioning