Advice for DMs: Monster workshop
This month's theme is Monsters, and to kick it off with a bang I'm going to take a look at how to keep your monsters exciting. I want you to think about a troll. Think about your players fighting that troll. What comes to mind? Maybe how it looks, maybe that rend damage, but probably: regeneration. I had the enlightening experience of playing with a group of new players not long ago, and I can tell you I've never enjoyed using a troll more. Because they had to fight it three times. They pushed it down a ravine, a fall they knew should kill nearly anything. Hours later they found it stalking them across the grasslands, and they ran it through with a dozen sword blows and an entire brace of throwing knives. They took off and that night they hid their camp, went without a fire and huddled up for warmth and courage. And the troll returned. They battled it fiercely and finally the party magus happened to harm it with fire and the players observed that its healing seemed to stop. That night they sat by the smouldering body of a troll, watching its burning husk....just to make sure.
But what does that do for you? Your players /know/ that fire kills trolls. If you threw a troll at them it'd be torches and fireballs immediately. At best you can make them roll knowledge checks to see if their characters know about the weakness, but it doesn't capture the same wonder. Two main components made the troll encounter memorable: Capability, and Mystery.
Capability is simple: What does your monster do? What Can your monster do? A quick glance at a monster's stat block tells you everything a monster can do, it's all there in black and white. Look closer. Really read that stat block, think about all their abilities. In my above example the troll threat would have been a single encounter had I not noticed the troll had scent. But that one minor sensory ability turned it into a relentless hunter instead of a random encounter they didn't technically kill. I can't tell you how many monsters can greater teleport at will, but I can tell you that the day I really thought about that was a great day for me. For a solid week my players were pursued and harassed by a Pairaka Div. It would pop in, stealth near them, wreck havoc with its charms and abilities, and then vanish by the time they'd stopped the horses and drawn steel. By the end of it they were sleepless, constantly armed, half of them were mind controlled and all of them had developed some very unhealthy fixations. Technically the div only had dimension door but it works equally well with the host of outsiders who canport at will. A cr6 monster ruined the lives of a 7th level party and my players never forgot that encounter.
Look at a monster's stats and abilities and think "What is the most interesting thing about this monster? And how can I best use all its abilities?". Even a player who has fought 100 demons will give pause when this new demon vanishes from combat and returns 12 seconds later holding the players childhood blanket that they left in their home town. It's easy to fall in a rut thinking about the tactical effects of every spell and power, but there's so much more to explore. Lots of monsters use swallow whole, but when a monster swallows the party mount, or cohort, or even just a lower level friend that ability becomes a race to get eaten and cut out the ally rather than just another slugfest. Take nothing for granted. Study that stat block for a few extra minutes and turn every ability upside down.
Mystery is a little harder to achieve. We all have that player who knows every monster, every ability, every tactics, and is never surprised by anything. The poor fools. The more you think you know, the easier it is to surprise you. The day that troll eats a face full of flaming acid and just keeps healing it will be your most knowledgeable player who is the most alarmed. There are thousands of templates out there that can mix a monster around and turn everything a player thinks on its head. Sometimes though a simpler change is fine. Swap out one signature weakness or ability with another. A regular looking troll that only takes lethal damage from electricity is simple enough to shake things up for a veteran player but you can go further. Any videogame enthusiast can tell you that sooner or later you're going to fight something with only one weak spot. As popular as this trope is we see it almost nowhere in DnD. Just take this idea: Throw a big scary dragon at your players and just make it immune to damage. Even magic swords bounce off of it, fire and lightning alike cannot wound it, it seems unstoppable. But when it breathes flame, for just that one moment it's mouth is wide open and it's throat exposed. Perception checks and knowledge checks draw this to the party's attention and it's up to them to decide who is going to get that close. Once they hit the weak spot it's an automatic critical hit for maximum damage. Just a few good blows like that can kill the beast, if it doesn't flee after the first one. A roiling "Hate elemental" draws power the more it's attacked but healing and spells giving bonuses instead cause tremendous harm. A quantum filcher teleports every time the characters look at it, forcing them to fight it blind...or work together to look in every direction at once, banishing it back into a state of existential uncertainty. All of these examples are memorable and challenging and yet each one is just a regular monster with a unique catch slapped onto it. This might adjust the CR a point or two, if you feel it's appropriate but usually it doesn't. A monster with a way to be beaten instantly is a fine trade off for beingimpossible or very hard to beat normally.
The two methods above work great to take what would be an old stale monster and make it something special. For some campaigns you can make sure that every fight feels like something new and bold even while using the same monsters as always. Other campaigns should also mix in more traditional foes that fight in a predictable manner, sometimes players just need some monsters to pound without thinking too hard, it all depends on your group and your campaign style.