Ideas

Rather than making a baddy impossible to hit, consider giving it mega-hit points.
Super hard to hit leads to great player frustration. Allowing them to do damage leads to a sense of accomplishment. another variation of this would be to give the bad guy a forcefield that shows damage.You could also give them a mastermind ability that sluffs the damage of to minions who should be easy to kill.Once in a while a super high to hit encounter is ok ,but it should be few and far between.

Your mileage may vary

A swamp witch that, when she dies, a huge black worm burst from her corpse and tries to burrow into the nearest living creature to use as a new host. If the the worm is injured in-between hosts smaller black worms swarm out of any wounds and burrow into anything living.

The players encounter an organization dedicated to toppling tyrannous governments; both good and evil. Secretly, they are led by the twin daughters of Lucifer / The Lightbringer / (Generic Adversary God). These daughters have an appropriate set of divine or demonic powers, but strictly speaking, are not evil.

You type the command into the console. 'Insufficient Guineapigs' the computer responds. Baffled, you retype the command. 'Insufficient Guineapigs' the computer replies again. You decide you should start smaller, with a simpler command. 'Insufficient Guineapigs'

What does cycling the spaceship's airlock have to do with a now-extinct Peruvian rodent?

Fedolf, the notorious headsman of Iddland, is known as much for his beheadings as for his operatic arias of doom. A tower of power, standing nearly seven feet tall, and weighing in at almost four hundred pounds, Fedolf strikes fear in all onlookers, especially when he dons his executioner's hood, and goes shirtless, wielding his gigantic double-bladed pole-axe, on his way to the headsman's block. He possesses a beautiful singing voice, and will often send off his charges into the next life, while belting out baritone dirges and antiquated arias, usually involving death, destiny, and duty, in heavy doses.

At regular intervals, a god takes control of your world's weather. Choose the gods at random each turn of the interval and begin applying their effects subtly. Their power will depend on the strength of those who worship them.

For civilised people, worship of a god actually wanes during that god's month in an effort to reduce their potential destructive effect.

In some virtual forums going AFK is the equivalent of passing out drunk on the couch at a fraternity party. Everyone from hackers to everyday participants will take shots at you, and you can expect your avatar to look very different when you get back. Hopefully you will notice quickly.

Draw the party together as debtors to the local lord, king, head of the thieves guild, or mercenary clan, who is forcing them to work together to pay off their debts.

tuck a small mammel into this 2' box,carry it under your left arm and the little creature takes some damage for ya and some spells cast on you are misdirected to it. If you do't mind the muffled screams!

This hedonistic dragon,sets up camp in a cities alley exchanging his hot tasty breath weapon for strong drink.

In your world the pantheon is a rotating system of formerly human individuals. You have been selected for god-duty for the next 2,000 years. How are you going to get out of this?

An adventuress, Semma, has returned to her home village, but something has gone badly wrong: it's the furtive glances, the conversations that stop when she approaches, the childhood friends who now have no time for her. She returns to the town or city and enlists your party to help her find out what's going on.

The party must find out what is wrong: something (perhaps a Cthulhoid monster, perhaps a gang of vampires, perhaps just a bunch of bandits) is extorting obedience from the villagers by threatening their loved ones with at least death. Once the source is discovered, Semma and the party and a few brave yeomen (and women) must deal with the threat by finding its base and defeating it, and then deal with the remnant corrupted villagers who willingly served the Evil Force. These may prove to be the most difficult foe...

A casual remark by one of the party about something met on a previous adventure leads to a summons to and grilling by the Mages' Guild (or equivalent), probably resulting in an expedition back to that area to recover the things for study by the Guild. The party's presence will be 'requested' for this expedition.

A rare branch of the arcane masters, encountered only among the deepest hill folk or ramshackle cabins, the Saucerer takes his power directly from the consumption of cheap liquor. Only the strongest, rankest, most nauseating of homebrewed alcohol will do, where it is instantly converted into mana available to the caster. Without a minimum level of inebriation, the Saucerer will be unable to cast any spells, as focus inhibits his spellcasting abilities.

These humanoids have flesh that resembles a peeled orange or other citrus fruit: semi-translucent with prominent veins. Brightly colored, some Rutaceaens are orange, some are yellow, and some green. They prefer warm climates with lots of rainfall.

Arakis's zombie problem went largely ignored, because it was believed that the Shai-Hulud (sand worms) would effectively dispose of the undead walkers. Unfortunately the problem grew until the roving herds of sunburnt ghouls threatened the flow of spice. While observing one of the great reanimated masses over take a Harvester from the safety of an ornithopter, an Imperial Zoologists noted that the sand caked flesh eating revenants walked entirely without rhythm.

Shortly after receiving this report the Bene Gesserits claimed to have a solution to Arakis's hellish resurrections. They asserted to the agents of the great houses that the worms could be drawn to the walkers using an ancient sonic weapon. According to the sisters, exposing the zombies to an antediluvian ballad composed by the great master M. Jackson would force them to step in time.

Hu was an ambassador of the Seventh Emperor of the Reng Dynasty. Throughout his life he traveled across many miles and lands to entreaty with neighboring kingdoms and the semi-savages who dwelled amidst the Metal Mountains.

During one such diplomatic mission, Hu was gifted a small iron marble as a gesture, by a shaman of the Kiy-Kiy tribe. Little else is known of Hu, but that marble was lost and is now somewhere out there for someone to find.

A tiny, shiny sphere, the marble has several properties. First and foremost it is a strong magnet, considerably stronger than its size and density would indicate.

Secondly, if thrown or rolled upon the ground and the command word is spoken, the iron ball will magically enlarge to either the size of an ogres's head or to that of a great globe, twelve feet in diameter. The rolling ball of either size will continue to roll or fly at the same relative speed it was when launched as a marble, and can thus cause great damage to anything in its path. The magnetic power of the ball will also magnify when enlarged.

Legends claim that the ball has been tossed from besieged castles upon attacking foes and rolled at marching armies in ages past. At the end of such rolls, the larger size globe has been known to not only crush soldiers underfoot, but to also 'collect' many dozens of metallic weapons and bits of armor unto itself, appearing as an armored sphere, with swords and spears sticking out from it in all directions.

Owning this powerful marble has its drawbacks. Anyone carrying it on their person, will experience the iron ball's insidious effects after some time. The owner feels no worse for wear, but after two month's time they will suddenly awaken one morning to find that their hair has fallen out completely, their teeth loosened like baby's teeth ready to drop, and their fingernails simply shriveled and sliding off the fingers and toes. Perhaps unbeknownst to the owner at first, the iron ball also renders an owner sterile or barren by this time.

Regular clerical healing will not reverse this horrible malady. Only finding and beseeching a shaman of the Kiy-Kiy tribe to heal the iron ball's effects with their particular brand of magic, will work.

Hu's Iron Ball should be handled carefully by players and gms.

Ursula is a were-bear; she and Stewart, the bailiff of a rural feudal estate, are lovers, sharing all their secrets as lovers often do. Recently something has been taking livestock. *He* must deal with suspicions that a bear is involved (one has been seen nearby...): *she* must defend herself if it is revealed (perhaps by a jealous suitor, perhaps accidentally) that she is a lycanthrope. The adventurers might be called in to track the hungry beast(s) and discover her secret, but be able to reveal that the culprits are a small pack of wolves that has moved in to the area. The adventurers might earn the gratitude of both Ursula and Stewart, depending on what they reveal, and to whom.

With a huge number of sacrificial victims, another realm long ago secretly bound a Prince of the land spirits, Iorstonn by name, thus ensuring the fertility, and improving the magical defences, of their realm. But binding that Prince to one spot has disrupted the weather and magical patterns (ley lines?) over half the continent. Things have become bad enough that a coalition has been formed to rectify the situation, once divinations have established the cause.

Your party is one of the teams assembled to do the rectification: find the binding object and steal it ('so that we can destroy it here in our realm, of course'...*) or destroy it over there. Presumably the Prince will be grateful to his rescuers and angry at the binders, but with a greater spirit like Iorstonn one can never be sure: such beings are 'tricksy'.

* Of course, the rulers of our realm might want to control Iorstonn for their own ends...

The bronze Gladius Tyrvaard carries has been enchanted by the Spirits to be very heavy to any but the wielder. Tryvaard (or any others who use the blade) will find the sword light and easy to wield. Those on the receiving end receive a strike as if made by a much larger and heavier blade. For such purposes (damage, ease of parry, breaching armor), it strikes as if it were a two-handed sword of great size. The blade, being spirit-forged, is also extremely durable, well stronger then normal bronze.