Giant Spider & Spider Swarm

Monster-wise, it doesn't get more basic than this.  Back before solifugids and scorpions, before the red box or even the blue and white boxes, there was a Hobbit, a dark wood, and giant spiders.

Three sarcophagi contain what appear to be silk-wrapped mummies.  Closer examination reveals that they have actually been wrapped in fine webs.  Closer examination also disturbs the spider swarms within the corpses, who come spilling out to attack intruders.

A foolproof plan to invisibly sneak into an enemy base through the postern gate runs afoul of giant spiders.  The gate lies near the cesspit that is the spiders' primary hunting ground, and with tremorsense they don't need to see to spot potential prey.

Spiders are considered unlucky in the Jacinth Kingdom.  In part the reason is cultural—lunar eclipses are said to be the work of the Night Spider.  But there is a more practical reason as well.  The Jacinth Kingdom is a magocracy beset by div-touched kobolds, who specialize in using giant spider egg sacs as missile weapons and catapult shot.  The sacs burst on impact, releasing spider swarms that are incredibly effective at disrupting spellcaster concentration.

—Pathfinder Bestiary 258