[RPG] As a DM, how can I handle the Druid spying on everything with Wild shape as a spider

dnd-5egm-techniqueslost-mine-of-phandelverwild-shape

I GM Lost Mines of Phandelver.

My player's level 2 (soon 3) Druid has discovered that while in Wild Shape in spider form, they are almost impossible to catch, especially since in caves etc. they "fit in" (which I agreed to twice).

However, this results in the PCs knowing pretty much the entire dungeon layout since those are somewhat small.

What can I do to hinder this full exploration without being unfair?

I've tried:

  • Letting time run out. Wild Shape doesn't last too long, but 2-3 hours still suffices for almost all dungeons.
  • Have NPCs swat against the spider if they see it – which doesn't do much due to the high Stealth bonus, and by climbing on the ceiling she now keeps out of their sight/range.
  • Have NPCs change post from between her exploration and the party entering – which kinda works, but still leaves a lot of the dungeon-crawling mystery lost.
  • Have something happen to the waiting group where the Druid would've helped (aka classic anti-splitting stuff)

Any other ideas? I'm ready to amend the prewritten dungeons and maps with new ideas, but so far none came to mind (except maybe an antimagic field in the final dungeon, but it would not fit for any of the other encounters/dungeons before that).

Best Answer

Introduce a threat to the spider.

There are plenty of normal creatures that live in caves that pose a threat to spiders. Frogs, birds, and even other spiders are natural predators. You can introduce other things like snakes and NPCs specifically looking for prey. Certain spells like alarm may not care if it were even something as small as a spider that entered its area of effect.

The spider may get caught in a web, sap, or other sticky substance and lack the strength to break free without reverting to its normal form. This would put them in a predicament since they're within the cave without their team and have to expend further resources or may have alerted the enemy in the process.