Spider

A friendly spider is a familiar from Find Familiar (the spider is on the list), a druid's wild shape into a Tiny beast, or a witch's pet the party has coexisted with since the first session. The animal has 1 HP and Intelligence 1, so the framing is "tool that walks", not "companion".

The use case is reconnaissance. Walk and climb 20 ft. each means the spider scales walls the rogue cannot reach, slips through one-inch gaps under doors, and reports back through the familiar bond. Stealth +4 with Darkvision 30 ft. means it sees pitch dark and rolls well to avoid notice. Use it for scouting the next room, mapping the corridor, identifying patrol patterns. Do not put it in combat. AC 12 and 1 HP mean a sneeze kills it.

The texture is squeamishness. Half the table loves a spider familiar and half hates it. Play the comedy hard: the spider on the bard's shoulder during a noble's audience, the spider the innkeeper finds in the soup, the spider the paladin steps on and must apologize to the wizard for.

When the spider scouts something the party needed to know, narrate it in one clean sentence and move on. The spider earns its keep in two-line check-ins, not in extended scenes.

A spider is a CR 0 prop with 1 HP. The stat block gives you Tiny size, AC 12, walk and climb 20 ft. each, Stealth +4, Spider Climb (ignores movement restrictions on difficult surfaces and ceilings), and Web Walker (ignores movement restrictions from webs, knows other creatures on the same web). The Bite action deals 1 piercing plus 1d4 poison damage. None of this amounts to a real fight.

Treat the spider as scenery with mechanical options. The point is that the room had spiders in it, the players brushed them off, and somebody made a Stealth check at +4 because the spiders provided cover. The honest fight is no fight at all. Use spiders as a trigger, not a combatant.

The way to make spiders a threat is in volume and atmosphere. A single spider on a doorframe sets the tone for an abandoned crypt. A dozen spiders skittering across the wizard's bedroll while she tries to sleep is a Constitution save against losing a spell slot to interrupted rest. A nest dropping from the ceiling onto the rogue who disturbs a web is a Dex save against being startled into the trap she was about to disarm.

If a player insists on initiative, run it cleanly and end it fast. Any hit kills the spider at 1 HP. Move past the round and back into the fiction. The strongest play is restraint. The cave gets webs, the players get Stealth advantage from cover, the cleric finds an egg sac that hatches in the backpack three days later. The spider does its work as flavor and the table remembers the cave.