3.5e, Forgotten Realms campaign setting.
Sir Markus Kassul the Third was a son of a sorcerer, grandson of a sorcerer, and very, very poor at magic. After only mastering the most basic of spells, he found he could progress no further and took to learning swordplay from his father's bodyguard, to the embarrassment of his family. Figuring his son would come to his senses if he got the adventuring out of his system, his father allows the young boy to travel and experience the world, take a few hard lessons and realize the folly of his fantasies.
Unfortunately for him, he fell into the company of a band of adventurers as they were hot on the heels of tracking a cult of the dead god Myrkul, intent on reviving their evil deity. He traveled in their company to a remote location deep in the Evermoors where they had tracked a slaving ring before he realized his mistake. Foolishly he decided the wisest course of action would be to make a fast retreat to Silverymoon upon starting to receive supernatural visions compelling him in that direction instead. Bidding his allies farewell, he set off alone across an undead infested swamp.
Travelling cautiously availed him poorly, for he was set upon several times by wild creatures. However, upon the border of the swamplands, he met his worst fear: three shambling corpses stood arrayed before him, their life long having fled their bodies. However, these are no ordinary undead. They are the spawn of Myrkul!
The creatures have zombie stats except for 2 differences: a fear aura, will negates, and a free action ranged touch attack to inflict a worm that spends 1d4 +1 rounds burrowing under the skin, then enters the brain and slowly drains 1d3 intelligence per round until the player becomes a mindless zombie themselves. The worm can be killed by inflicting 1 point of damage to it before it enters the brain.
Our poor Markus reacts slowly to the advancing horrors, and two get the jump on him, one spitting a worm that strikes him in the face, the second attacking his horse. Both Markus and his steed fail their will saves against the creatures and flee in total terror. Within seconds, the worm that struck Markus has crawled under his skin and burrowed to the rear of his head, burning pain wracking his body as the creature burrows into his skull. By the time Markus recovers from his terror, it's too late.
DM: "As the haze of terror lifts, the burning agony you've been feeling ceases. Suddenly, you feel no more pain."
Markus: "I reach around to feel the back of my head."
DM: "You feel a hole, about the size of your finger, that penetrates through your skull. There's a trickle of fluid leaking from it." *dice rolls* "You also feel your mind start to cloud and thinking becomes difficult for you as you lose one point of intelligence. What do you do?"
Markus: "I attempt to use Mage Hand to pull the worm out of the hole."
DM: "Okay, make a Spellcraft check, DC 25."
Markus: *dice rolls* "I fail."
DM: "Your spell discharges and you attempt to pull the creature from the hole, but you are unable to get a grip on it. At the start of your next turn... *dice rolls* "you lose another two points of intelligence. What do you do?"
Markus: *suddenly flicking through character sheet* "Um, I'd like to command Sarah to go inside the hole and grab the worm."
DM: "Your familiar?"
Markus: "Yes."
DM: "What sort of familiar do you have?"
Markus: "A tiny viper."
DM: "Okay, make a diplomacy roll to convince your familiar to enter your skull, DC 15."
Markus: *dice rolls* "I make the DC by 1 point."
DM: "Your familiar Sarah, against her better judgement but out of willingness to save your life, enters the hole created by the worm. Given the lack of any indication where the worm is and extremely confined space available, I'll need you to make an attack roll for your familiar against the worm, AC 30."
Markus: *dice rolls* "My familiar rolls a natural 20."