One issue I've found in 5E is that encounter design assumes players win, every single time. This tends to create situations where players might feel a bit of tension, but there's very little at risk. Theoretically a PC could die, but unless this creates a cascade effect towards a TPK, this just annoys the player in question. The most memorable combats aren't fair to the PCs; they should be outnumbered and outgunned. Some suggestions:
- If you use the DMG guideline for encounter design, make the battle Deadly x3 at least. This should give you plenty of enemies to work with, and the battle is more balanced between both sides.
- Use more than a single monster, even if it's legendary. Minions (4E style or not) are good to swarm PCs, particularly squishy back line characters.
- Have a mix of enemy types. Everyone being melee is easy for the party to counter with control spells and abilities.
- Having a spellcaster is a really, really good idea. They generally throw off PC tactics with control spells, and healing is particularly annoying. Unfortunately with the new direction of spellcasting, this may no longer be a useful tactic.
- Don't just use the actions in the stat block. You can give humanoids different weapons, such as nets, polearms, crossbows, etc. Have flying monsters grapple PCs and attempt to fly off with them (leading to falling damage if the monster is killed). If you have a lot of weaker monsters, have them grapple and shove prone a character. With the Help action granting advantage, eventually this will work. Then the PC has to decide to give up their entire action to break free or suffer the prone penalty.
- The environment fits in here, since using it to the enemies advantage is optimal. having ranged attackers with both cover and nearly inaccessible forces the party's ranged attackers to deal with them, instead of helping focus fire. Best example was having them up a 100 ft high cliff shooting down, after which they moved 15 ft away from the ledge. The PCs had to ready actions to attack (negating multi-attack) or waste long range AoE spells without knowing if they'll hit anything or not.
- Focus Fire isn't just for PCs. Even if the party has tubthumbing, once a PC gets to that point, only 1-2 enemies is all it takes to keep them going down.
- If you want to strike fear into the hearts of the players, use Focus Fire and attack downed PCs. This will likely lead to at least one PC death, and will often lead towards the death spiral to a TPK.
- If you really want to add tension, throw AoE spells that damage downed PCs. This puts them in the scary position of rolling a "1" to die before they're healed. Usually when this happens, unless a healer is before the downed PC, another PC will stop to feed them a potion of healing. This loss of an action slows the party down, and is really inefficient, since the PC might go back down again soon.
