The Inexorable Beast

Some people call it the Questing Beast.  It quests for you.

The fools have opened the Book of the Beast and its curse has fallen upon them.  There is nothing that they can do.  Though the Inexorable Beast can be killed, it will always return, stronger and more horrible than before.  It will pursue the cursed individuals to the edges of the universe and beyond.

Basically, the beast is an intractable guardian of some arcane secret.  Once the PCs have become victims of its wrath, it will continue to hunt them until they are all dead.  The first time the PC's open the forbidden door, or read the black runes, or pass through the halls of the dead, the beast will appear for the first time.

Starting Form
HD    high enough to shake up your PCs
AC    14
Atk    bite (1d6+1 damage)
Int      animal
Mov   a little slower than the average dude
Save  15 - HD

After it is killed or evaded, the beast vanishes, leaving behind nothing except a bit of blood on the PCs' weapons.  1d20+4 hours later, the beast will reappear in the worst possible place.  (If this seems too harsh, slow it down.)  If they flee from it into a cave, they will find it waiting.  If they fill a bathtub with water, the Beast will come leaping out.  If they check under their bed for monsters, the Beast will crawl out ten minutes later.

If the PCs escape, that counts as ending combat (it doesn't track them, it just reappears later).  If the beast kills a PC, it will likewise disappear with a triumphant roar into a cloud of acidic ash and sulphur.  If the PCs trap it somewhere, it will eat itself and disappear with a frustrated mewl.  If the PCs split up, the beast will go after whichever group seems more vulnerable based on how each PC fought in the last encounter.

Each time the beast reappears, it gains new mutations that make it bigger and more horrible.  It gains three of these each time: a size increase, deadlier attacks, and another horrible ability.  (If this seems too steep, slow it down.)

1. Size Increase.  The Beast gains 1 HD.  After gaining +3 HD the beast is big enough to hold people in its mouth with a successful bite attack, doing automatic damage on subsequent rounds but making other attacks at a -3.  After a gaining a total of +5 HD the beast is big enough to swallow people whole instead.

2. Deadlier Attacks.  Flip a coin.  On a heads, the DM chooses an attack mode of the Beast to incease in potency and do +1 damage forevermore (or +1d6 damage, if you have a high level party).  On a tails, the Beast grows a new attack mode.  The DM should choose this new attack based on whatever would erode the most of the players' advantages based on their tactics in the last fight.

Like, if the PCs killed it from range last fight, then it will grow a spike shooter.  If they surrounded it and killed it in melee, then it will grow the acid spit so it can catch more of them in the cone.

Here are six options: 1 claw, 2 bite, 3 spike shooter (50' range), 4 spit acid (15' cone), 5 horn/spike, 6 tail/flagella (50% chance of grabbing/disarming).  If you can't decide, roll a die.

3. Other Horrible Abilities.  Lastly, the beast grows a new ability.  The DM should pick whichever one would be the most horrible for the PCs, based on their tactics in the last fight.  Like if they escaped it by flying away last combat, there is a 100% chance that the beast will grow wings the next time they see it. Subsequent enhancements to the same ability are indicated with /slash marks.

1. More legs.  The beast becomes as fast as a man.  /faster than a man. /faster than a horse.
2. Discerning tongue.  By licking the air, the beast can smell hiding or invisible creatures with 40% success.  /80% success.
3. Freakish agility.  The beast now has 16 AC.  /17 AC.  /18 AC.
4. Jellied flesh.  The beast now takes half damage from bludgeoning weapons and +50% damage from slashing weapons.  /no damage from bludgeoning and +100% from slashing.
5. Thick hide.  The beast now takes half damage from slashing weapons and +50% damage from piercing weaopns.  /no damage from slashing and +100% from piercing.
6. Armored scales.  The beast now takes half damage from piercing weapons and +50% damage from bludgeoning weapons.  /no damage from slashing and +100% from bludgeoning.
7. Spiderlike Limbs.  Its arms become as long as pikes, allowing it to attack as if with a reach weapon.
8. Anti-magic properties.  Its skin becomes polished and nacreous, and all magic has a 2-in-6 chance of failing when used against it.  /4-in-6
9. Wings.  The beast can fly.
10. Amphibious.  The beast can swim and breathe underwater.
11. Bludgeoning head.  The beast can knock down doors in a single action with a 3-in-6 chance of success.  /5-in-6 chance of success.
12. Giant brain.  The beast's features become more human-like.  It gains near-human intelligence, the ability to speak a few words, and the ability to cast a 1st-level spell (choose the most effective one).  /human intelligence, fluent speaking, and a 2nd-level spell.  /genius intelligence, mastery of language and all the noises of hatred, and a 3rd-level spell.
13. Gaseous Form.  The beast can now slither under doors and creep in windowsills.
14. Explodes upon death.  Does damage = half HP, save for half.
15. Poison.  Save or take half of your HP as damage.  /save or die.
16. Mimicry.  Can imitate noises, shapes, and colors.  (Like a mimic, but sans glue.)
17. Burrowing.  Moves about as fast as a crawling man.  /as fast as a running man.
18. Invisibility.  It's fucking invisible.  /if you actually look at it via see invisibility or something, its new horrible form will drive you temporarily insane unless you succeed on a save.
19. Elemental.  Takes half damage to an element (fire, ice, lightning, acid) and +50% damage from its inverse (ice, fire, acid, lightning, respectively).  /immune to damage from an element but takes double damage to its inverse.
20. Thrashing, bladed tentacles.  All creatures within 5' automatically take 1d4 damage each round.  /2d4 damage each round.

Again, if you can't think of what horrible ability to give it, roll a d20.  But if you roll for an ability that wouldn't give it an advantage, based on what the beast knows from the previous battle.  It is an incarnation of Lamarckian evolution after all.  Like, don't give it water breathing unless it's an ability that the beast "wished that it had" in the last fight.

The beast adapts.  The beast doesn't anticipate.

The beast doesn't speak, although if it could, it would probably be frustrated by it's attempts to convey the ineffable depths of its hatred.  Like, humans can love in ways that salamanders can't imagine, and the Inexorable Beast can hate in ways that we cannot conceive of.

You'll notice that some of the damage types (slashing, bludgeoning, piercing) and the elemental types (fire, ice, lightning, acid) give the creature weaknesses.  This is intentional, since one tactic that the PCs could use is to intentionally use a lot of clubs on the beast one fight in order to be able to slaughter it with spears on the next fight, after it grows its Jellied Flesh. (This is the sort of information that they could find out by consulting a sage, or learning about previous people who have been hunted by the Inexorable Beast).

Describing the Beast

Honestly, just go wild.  Don't go too wild, too fast, since the beast needs to get bigger and presumably more complex each time it comes back, but just keep adding limbs and jaws, and faces.

Especially add faces.  More faces.

I imagine its starting form looking like a cross between a hairless wolf, a pug with a skin condition, and a human.  But it could be anything, really.

Winning the Beast

You probably should give your PCs a way to kill it.  Maybe after traveling to a nearby city/castle and browsing the library (and giving the beast a chance to wreck the library) they learn of a method.  Suggestions:

1. Kill the beast according to a very specific recipe (a damage password).  E.g. two stabs from silver daggers and then a fireball spell.  The fireball spell must be the attack that delivers lethal damage, and nothing else can damage it between the two dagger thrusts and the fireball.

2. Kill it in consecrated ground.  A church, or holy site.  (Of course, you've also got to convince the people in the church that letting you in is better than just locking  you outside and letting the beast kill you.

3. Collecting samples of its blood and creating an Anti-evolution toxin, that will make the beast devolve.  Losing 1 HD per round until it is just a harmless little zygote.  After it petrifies, you can put it on a chain and wear it around your neck, granting you the powers of whichever mutation the PCs hated the most.

4. Getting it to evolve into a specific state and then imprisoning it.  E.g. killing it with acid twice, so that it turns into a beast of sloughing, corrosive stone, and then burying it beneath holy dirt (made the same way holy water is).

5. Passing off the curse onto someone else.  Get them to accept the book.  The ole switcheroo.  This may involve going back tot he exact place where you got the book.