Shrieker Fungus

A shrieker as ally is a deliberate alarm, planted by the party, a druid, or a fey patron in a tunnel mouth or campsite perimeter. The shrieker is a stationary plant with a 5-foot walk and no ability to choose what it shrieks at, so the "ally" framing is really about whether the party owns the territory it sits in. In most campaigns this means a druid grove, a myconid colony allied to the party, or a Underdark safehouse the party has cleared and dressed.

Plant the shrieker at the chokepoint. The party stages it inside a 30-foot zone where any uninvited foot or torch trips the alarm. The party walks past it daily; their Stealth and lack of Bright Light keeps it quiet (the shrieker triggers on movement within 30 feet, so any visitor sets it off, which is the point). The shriek itself is audible 300 feet, which means a shrieker in a cave entrance wakes the party sleeping in the back. That is the whole utility.

The shrieker does not fight, will not move on command, and dies to one stray crossbow bolt. Treat it as terrain the party maintains. A myconid handler can move it slowly (the colony shifts the patch over hours), and a druid with Plant Growth can multiply the patch into a real perimeter. Use the patch as a recurring touchstone: the party returns to the safehouse, the shriekers stay quiet, the party knows the lair has not been breached.

If the shriekers are silent the night the party most expects to need them, that is the scene. Something got in past the alarm. Roll for what.

A shrieker is not a monster, it is an alarm bell with hit points. AC 5, 13 HP, walk 5, no attack, no damage, immune to Blinded, Charmed, Deafened, and Frightened (it has no senses or mind for those conditions to land on). The whole stat block is one reaction: Shriek, triggered when a creature or a source of Bright Light moves within 30 feet, audible for 300 feet, lasting one full minute or until the shrieker dies. Run the shrieker as a dungeon trap that costs the party time and stealth, not as a creature that fights.

The setup is the encounter. Place shriekers in clusters of three to six in a tunnel that connects to whatever the party actually needs to fight. The shrieker patch is the lock; the wandering monster table is the door. When a shrieker goes off, roll on whatever your dungeon's reinforcements look like (drow patrol in 1d4 rounds, hook horror in 1d6, the duergar shift change in 2d4). The party hears the shriek, knows it has been heard, and now has to choose between sprinting forward and hoping or backing out and trying again.

Counterplay matters. Shriekers fall to one cantrip of any kind, but you have to be in their threat zone to swing. A rogue who Stealths in (Passive Perception 6, blindsight 30 means stealth above DC 10 walks past the threshold without triggering) and bonks the shrieker dead in melee solves the patch in three rounds without a sound. A wizard with a torch has just lit every shrieker in the room and is now the problem. Telegraph this. Put a dead torch and a chewed bone at the cave entrance.

The shrieker is the GM's gift to slow play. Use them when the party is sprinting through a dungeon faster than the dungeon deserves. One shriek, one wandering encounter, and the party remembers that the Underdark has rules.