Bezoar

The Bezoar was the chief antagonist of the Buffy episode "Bad Eggs" and had plans to enslave the population of Sunnydale so that they could dig it out of its apparent imprisonment deep under the basement of Sunnydale High School.

History
The Bezoar apparently lay dormant under Sunnydale High School since prehistory but awoke at some point in recent history, through unknown means one of its spawn managed to take control of a health teacher, who collected more of the creature's eggs and handed them out to students by unwitting staff as part of an experiment in which students were encouraged to treat the eggs like babies - much to the horror and disgust of many of the students.

These eggs proceeded to hatch into parasitic monsters similiar to the face-hugger form of a xenomorph - they latched onto the spine of their victim and transformed them into mindless drones, who immediately began to head to the basement of Sunnydale High in order to dig the Bezoar up using a variety of mining tools.

The only students and staff not to be effected were Buffy Summers and Xander, as Buffy had killed the parasite that attacked her in her room and Xander (in typical Xander-fashion) had already boiled his egg beforehand, unaware it was the vessel for a monster - and getting a rather unpleasant surprise as a result.

In the end Buffy managed to infiltrate the basement of Sunnydale High by pretending to be a drone, then attacked the Bezoar via allowing the monster to drag her underground via one of its tentacles, she then proceeded to slay it: with the Bezoar's death all the parasitic offspring also died and the student body of Sunnydale returned to normal.