Gilladian
Adventurer
I would fully expect certain types of monsters to smash any spider they see. Orcs or goblins, for example, might easily grab any spider they see, pull the legs off it, and have a tasty snack. Human cultists would probably just step on it, or smack it with gloved fist. I have a friend who is absolutely bonkers about spiders. She'd smash every one, even if it was an eighth of an inch across, and even if there were dozens of them. Except she'd never GO there, to begin with.
Also, the "thing that eats spiders" would be a problem in many caves and natural environments. Even just other spiders.
However, it is also a fair use of a familiar. Just remember that a quick scout of a room by a nonhuman creature isn't going to reveal everything, and may reveal totally useless things. "There are three humanoids in the room. What are they? Ummm... I don't know the difference between orcs and elves!" and "wow, I had no idea that round thingy floating in the corner was a monster! It didn't shoot rays at me!" And "there are two clouds of fruit flies, sixteen cracks in the floor large enough for me to hide inside, and five suspicious holes in the wall where predators could lurk. How big? Ummm... big enough to eat me!".
Also, the "thing that eats spiders" would be a problem in many caves and natural environments. Even just other spiders.
However, it is also a fair use of a familiar. Just remember that a quick scout of a room by a nonhuman creature isn't going to reveal everything, and may reveal totally useless things. "There are three humanoids in the room. What are they? Ummm... I don't know the difference between orcs and elves!" and "wow, I had no idea that round thingy floating in the corner was a monster! It didn't shoot rays at me!" And "there are two clouds of fruit flies, sixteen cracks in the floor large enough for me to hide inside, and five suspicious holes in the wall where predators could lurk. How big? Ummm... big enough to eat me!".