Another in my series on trick monsters - today, it's the ones you just can't kill off. At least, not easily.
Some monsters are tough beyond toughness. They just can't be stopped through the traditional methods of the dungeon delver and monster hunter - straight-up damage. Fire and the sword might slow these monsters down, but it won't keep them down.
These monsters just won't die easily. Lay in some linament, because your sword arm is going to be sore before you're through.
But unkillable doesn't necessarily mean invulnerable, although it can - some of the magic creations in Glen Cook's Dread Empire series were literally invulnerable, and could only be carried off so the weren't a proximate threat.
These monsters can't be killed. Ever. Period. Nothing will drop them, or if it does, they instantly and immediately get better, like some monstrous version of Madcap. A good example of this is Cthulhu. Nuke Cthulhu, ram a ship into his head, whatever - you're just buying some time. They come back Phoenix-like to avenge themselves.
They may not even be able to be harmed. This might be sheer strength - a Rolemaster Black Reaver, for example - that makes damaging it difficult to the point of futility. Or simple immunity to damage.
Using these is tricky - they need be limited in some way, or at least foil-able, or they're simply not fun as well as not fair.
Some monsters can be killed, but only through a specific Achilles' heel.
The Sword-Spirit in Dungeon Fantasy Monsters 1 is one of these - it is both tough and only killable with a specific type of weapon. Or a specific weapon. They're cursed and thus are only vulnerable to breaking the curse.
Trolls can only be killed with fire or acid, although I know some read D&D's rules as saying if you kill them before they start regenerating they are dead, dead, dead. Where is the fun in that? Require the fire, I say.
Vampires can survive almost anything but sunlight and holy water.
And where do you even start on Demi-Liches?
The trick with these monsters is finding out what can kill them, and then using that to finish them off. It can be as simple as "magic weapons" or "fire" or as complex and fiddly as "a sword of silver, thrust into its heart under the light of a blue moon." Pretty much any monster that is unstoppable until you find its weakness is a Limited Vulnerability Unkillable Monster.
Some monsters can be killed, often with anything that would kill a normal creature, but get well afterward.
Typically the threat isn't that the monster is unkillable so much as requires multiple killings.
Some might have Extra Life and just get back up. Liches might have spare bodies to move to, gods might only have material forms you can mess with - but you're merely pushing the problem to another day.
Dungeon Fantasy Monsters 1 also introduces the "Psycho Killer" prefix, which pretty much makes killing the monster an exile in deja vue. The Golem-Armor Swordsman from DF is the same - you have to kill them more than once to keep them from killing you.
The Lord of the Maze in my own version of the Caves of Chaos is an Unkillable Monster. You can put him down, and the PCs have done so, but he keeps coming back. They have theorized his magic spear and armor are cursed and keep bringing him back. Whatever the truth, simply applying damage to him until he stops coming isn't the answer.
One nasty version of this kind of monster isn't a monster that refuses to die or just returns, but rather turns into something else. These monsters essentially use death as a morphing trigger to come back, usually in a stronger form.
This can be return as undead, physical transformation, or ascension into some higher form. While monsters can die and come back weaker, that risks them being irrelevant - if you could kill their "tough" form, who cares about the weak form? Having them come back stronger means that there is a resource-crunch. You need to kill them, but have enough in the tank to kill them again once they're tougher.
If they transform unless killed, you have a time-crunch instead.
I had some of these in my last campaign. Inspired by a the GURPS Knack Tattoo rules, I had a group of nasty orc assassin who were inscribed with a zombie ritual - so they'd come back from the dead, immediately, as a form of super-zombie. If the zombie was slain, well, they had a Skull Spirit tattoo, as well - and a ghostly assassin would come to avenge them on their slayer 24 hours later. Killing them ultimately was the solution, but their original form was perhaps their weakest one.
Unkillable monsters can be fun, although they can equally be frustrating. Some players won't spend a lot of time trying to find their vulnerability or seeing is a second crack will finish them off. They'll simply try to find some clever non-death solution (Entombment, say, or walling them into a room). Keep that in mind when you use them - they invite an indirect solution. This in and of itself can be pretty cool!
Okay folks, what Unkillable nuances and tricks did I miss? Many, I'm sure!