Mitch

This is off the cuff and written from bed so it’s gonna be shorter than normal and maybe not as well explained but I have an immediate answer!

The rpg monster I feel is most underutilized isn’t a single monster or even a single type or clade, but a category: the ageless.

This is a wide category, and I don’t know if anyone else uses it but it’s my own categorization: creatures who, through innate condition or outside impact, are functionally immune to the ravages of time (at least, as seen through the lens of our protagonists, whether that lens is modern human scale or is itself extended somehow). This may also come with “immortality” in some form: the daoist immortal is unkillable outside certain specific conditions, after all. It may not: some kinds of vampire are not only killable, but will either and starve without sustenance. “Ghosts” are exempt from this category by default, though “beings of spirit” aren’t automatically excluded.

“But Mara, a vampire is literally the central villain of the most successful rpg adventure product ev-“
Strahd is a brutish lout whose powers are all “mind control, terrify and smash”. He could be completely replaced with a 50-year-old human sorcerer that had come into physical power as well as magical.
“But what about the whole obsession with the reincarnating soul of his lost love” he can be wrong. He can be gaslighting anyone who gets close. He can truly believe this himself. Who cares. Who the hell cares, “I am Ancient, I am the Land” is a line raw as hell and nothing reflects that.

I’m being overly harsh. He’s picked up tricks along the way, sure. No one can say his name in his domain without him knowing and listening in (though again, age just gave him the opportunity to make sure it was set up, there’s nothing stopping a powerful 21-year-old wizard from doing the same thing). He has generations of terrible history to add weight to his actions (but a savvy warlord with a decent retinue could easily style themselves after a mythical ancient villain and capitalize on the same weight until it centered around him instead). He has collections of powerful artifacts and devoted servants and… you see where I’m going here, his age is used mainly as an “excuse” to have these things, to have all these things. And that’s fine! It works. It’s a GOOD excuse.

But that’s the thing, isn’t it? “Oh but they’re ancient, they’ve been around” is used mainly for monsters that are either decrepit, or colossal beatsticks. In published material - I do not know what anyone does in their home game unless they tell me somehow, maybe there is a thriving trend of MILLIONS of people who use this category skillfully, I have no way of knowing - in published material, outside of like, Unknown Armies, there’s such an obsession with enabling the player avatars to be the martial heroes, the potent magicians, the clever jacks and twisty jills, that they’re allergic to even giving us a Vetinari. No, the players must be able to Win and win Decisively with their trick that the monster cannot see through; it’s frustrating if the monster gets away and so you mustn’t set up a situation where it can auto-escape, that’s cheating; the players are proud of their plan and so you mustn’t undermine it even if the undying elven spymaster has seen this ploy 10,000 times across her career and has a playbook response and then a response to the players response to her response on speed dial. There’s no room even for a monster to engineer things so that no matter who wins, they don’t lose (or maybe even they win regardless of the outcome, or at least further a plan, or…)

David Xanatos is unwelcome outside of his cartoon series

And so the ageless are reduced to angry lumps of terrible meat. A dragon is a big lizard that can wreck your day and hoards gold and breathes fire whatever elemental breath WotC has codified this color of dragon breathing I guess. The ancient Strahd is a physical and magical monstrous powerhouse but can’t counter anything not on his character sheet.

I live by a “no gotchas” creed as a DM. It would be easy for me to silver bullet counter literally anything the players throw at me, if I desired. I have the power to alter the deal at any point, if I’m willing to borrow against my established credibility to do so. But for the most part, no, I do not LIKE waiting for players to make what looks like a good move and then activating my hidden trap card with a smug “gotcha” as they are now caught in a combo loop that exploits vulnerabilities they have no counter for

…but I make broad exception for the ageless, for monsters that whether or not they’re “smarter” than me, are certainly more experienced than I, the person running them. The ones who should know better. The ones who DESERVE better than being reduced to a D&D statblock and encounter.

There’s definitely a lot more but I don’t know if I can do it justice typing ad hoc on my phone.

I love how you are like "i'll be brief" and then do nearly a thousand words.

first off shouts outs for the xanatos reference, we love the king of heels. but digging into the meat of this answer, i tend to have this switch in my head where the gloves come off. in combat games there is a level of obstraction where you just kinda got to let it happen- the wizard can always perfectly place the center of his fireball so it hits where he wants. but it's equal for both sides there and it works if you don't think about it too much. but there tends to be that type of "no metagaming" rule at tables where you are trying to be in character and viewing things as a character and not as a person thinking tactically.

this is also largely a rule that gets thrown at the GM as shackles to keep them from killing their players. and like- honestly I think they are necessary training wheels until you learn how to run games. but frequently you are playing with a hand tied behind your back for narrative convenience.

ageless monsters rule to me because that is you getting to fucking go off. like, i'd tend to be nice and warn my players with like "hey just so you know, the boss is gonna be rough this time" and let them get into the right mindset. because these games are from wargames, and the fun of those games is taking asymmetric forces and smashing them against each other. you are getting to play tactically and have fun. and modern convention says, "okay but you should let them win." so it's just so fucking freeing when you get to say, no this person is going to play smart and go at them equally.

i get why this is a thing. @topplethrones and I were playing Undaunted Stalingrad the other night and I got a series of lucky rolls and got a Full Rout in 2 rounds, forcing a victory. So we had a silly, quick game and are both able to laugh it off and shake our fists. It can suck if that is the culmination of months of play. So I get why you don't always do this. But I rebel against the idea that you never should.

I don't apply to a "no gotcha" policy in my games- I love secrets, I love traps, I love setting something up beforehand to fuck with them. And I think that's the key. I am planning beforehand and don't know what they are going to do, if they step into then they take the consequence. But if they go a different way through that I didn't defend against, I'm not gonna conjure up some new trap/what have you. Which I think makes it work at my table because my players know I had this all figured out in advance and if they do step into something they know it's not me just choosing to fuck with them at that moment.

This is why for HbH, I made the Overlords what they are. Like here are four big adversarial factions with a leader for each. They grow with you so you will naturally cut them down over the course of an adventure leaving one who will naturally become the final boss. But the thing about Overlords is they all have a reason to BE the leader of their factions. So you have four foes in the game where, as the creator, I say "Go nuts".