Everyone knows that. Cut off its head. No, wait, cut off its head, stake the body through the heart, bury the head across a river from the body so that they'll never find each other.
What about dragons?
Some creatures get unique death conditions. You can slice a vampire to pieces but it will rise again unless certain obscure deeds are done.
In the early days of Creation, the Authority's first creatures were immortal and invulnerable. As He grew wiser, He made creatures weaker and weaker. How many ways are there for a mortal to die? Elves are untroubled by age or disease. The serpents of the earth will never age, never rot, and cannot be harmed by mortal weapons. Every type of powerful creatures must be killed or handled in specific ways lest it rise again. The ideas below might not be common knowledge. Seek out a Dr. Van Helsing or a village elder.
Feel free to add or suggest more creatures in the comments.
You can kill a mortal creature with almost anything: cold, disease, drowning, even fright. They are fragile. If the creature is left to rot, disease spirits may erupt to infect the air. If the creature was strong-willed and had unfinished business in Creation, they may rise again in a new form. Ghosts are the souls of creatures that don't know they're dead, or are too insane to care. Dybbuks and other embodied dead are created when a creature knows it is dead but has the strength to carry on, at least for a time. Mummies and wights and similar creatures are formed when the soul is carefully prepared and trapped before death.
|Sulphur Dragon, Yohann Schepacz|
You can kill a dragon with blades and spells, but that is not enough. In Creation, the body is merely a housing for the spirit and dragons are mighty spirits indeed. Cut them apart, make armour of their scales and trophies of their bones, and they will wait patiently. A year and a day after its death the dragon will come roaring back to life, reborn from a volcano, twice as strong and powerfully angry. Anything made from its former body will crumble into toxic ash.
To permanently kill a dragon, cut out its heart and lock it in a stone chest. Sail at least one day away from land. Toss the chest off the edge of the ship and let it sink into the cold deep. Do not think about the Blue Dragons that lurk in the deep. Dragons, being solitary and proud beings, aren't likely to resurrect their kin idly. Nobody knows how deep blue dragons can dive, or how deep the ocean really is. Perhaps the hearts fall all the way to hell. Some people think this might be a ploy by the blue dragons, and that one of their kind is growing more powerful every year by devouring the hearts of its kin. Perhaps, with enough, it could regain the lost power of the Sunlight dragon. Alternatively, paranoid scholars insist the blue dragons might be hoarding the hearts, ransoming their own kind or preparing a massive draconic army. It is said that there are volcanoes under the sea. Perhaps there's something else in the oceans that wants dragons kept out of the equation. Dropping the stone chest overboard could just be the first step.
Any blade can cut an owlbear, but age will never claim them. Disease nourishes them like ambrosia. Kill and owlbear and burn the body immediately or its tar-like flesh will birth a horde of eggs and larval owlbears (horrible little grub-things with beaks and too many feather-legs). The grubs remember who killed their parent and will, once fully grown, perpetrate a hideous revenge.
|Drugula, Alex Brock|
The peasant elder knows of three varieties of vampire. The first is created when the corpse of an evil man or woman is left unattended immediately after death rites have been performed, or if the rites are performed incorrectly. The body will rise and seek the blood of the living. It possesses terrible strength and a host of supernatural powers. To prevent this, criminals and suspicious travelers are buried decapitated and chained in some parts of the world. Decapitation is the only certain way to prevent them from rising.
The second variety is created whenever a vampire bites a living mortal. Their victim falls into a waking nightmare and swiftly dies of fever. No matter how virtuous they were in life, they will rise again as a vampire if their head is not removed.
The third variety, and the most terrible, is created when a truly evil being dies and wills itself, fully consciously and with appropriate magical precautions, back to life as a vampire. This is no accident or trick of fate. The new-born vampire is aware of its own strengths and weaknesses and is much harder to kill. The head must be severed and carried across running water, far from the body, and buried with a silver coin in its mouth. Full funeral rites must be performed over both the body and the head or they will rise again. Cremation is not an option, though few will ever speak of why. Fire elementals cower before a vampire.
Creatures so ludicrously magical can be difficult to kill, but once dead, many adventurers believe a Beholder poses no threat. This is a lie. The magical power that fuels these strange visitors from the Mirror Realms must be released. Harvest the beholder's magical eye-lenses and craft them into spectacles or wands. Use its skin to bind a spellbook. Do something with the body that exposes it to continuous magic or, in time, it will regenerate itself and seek your death. Or it might explode and take a village with it. Beholders are unstable. Treat a desiccated beholder corpse like unexploded ordinance.
Split each piece laterally, down the middle, or it will grow into a new purple worm in a few days.
It is impossible to truly kill a phoenix. They are one of the Authority's first creations. They eat nothing. They are made of pure white flame, the flame of magic crinkling the fabric of Creation. If slain, the phoenix is reborn in flames. It is a peaceful and harmless creature, unconcerned with mortal life or the intrigues of Creation.
But the phoenix can be captured. Kill it, and quickly trap its body in a metal shell. Iron won't do. You need strange alloys forged by creatures who have never seen the sun. Seal it in tightly. Lock the shell around it. Sink it in a deep pool of cold water. When the phoenix awakes, it will struggle, but the heat of its existence will permeate through the shell and into the water. You could build a primitive hot spring, but that is not the only purpose. Some say that cities deep underground are fuelled not by wood but by hot water drawn over arrays of trapped phoenixes.
Cut off a troll's limb and, unless reattached to the main troll, it will grow a tiny head and tiny limbs and eventually reach full size. Only acid or fire can slay a troll forever. Before ancient bargains were struck with acid and fire elementals trolls ruled over all Creation. There are huge oozing layers of troll bones and troll flesh, deep beneath the earth, below the vampire strata but above the heat of Hell. Entire underground kingdoms have grown wealthy on troll-oil.