Bat

A bat is a familiar, a vampire's spy lent to the party for one night, or a druid's wild shape used for reconnaissance the rest of the party cannot do. The Blindsight 60 is the entire point. In a lightless cave or a magically darkened room, the bat sees what nothing else does and reports back to whoever sent it.

Run the bat as a scouting tool, not a combatant. Fly speed 30 ft., walk speed a useless 5 ft., 1 HP. If anything notices the bat, the bat dies. So the play is: send the bat down the corridor, ask the GM what it sees, pull it back. A familiar bat can use the Help action via the familiar rules to grant Advantage on an attack against an adjacent enemy, which is the only combat contribution worth attempting, and even that gets the bat killed if the enemy has a reaction attack available.

For the druid wild-shaped into a bat, the calculation is the same with a safety net: the druid drops out of the form when the bat hits 0 HP, taking no damage themselves. This makes the bat the scout shape of choice for a druid who has been told the dungeon is dark. Echolocation through stone walls is not a thing the bat can do, but echolocation in pitch-black open chambers absolutely is, and the table moment when the druid describes a room nobody else can see is worth the spell slot.

Tell the player what the bat sees in second person, present tense, like a dream. The shift in cadence sells the perspective change.

A single bat is not an encounter. It is a sound, a shadow across a torchlight, a thing the rogue almost decapitates before realizing it was a bat. With 1 HP and AC 12 it dies to a thrown rock. The reason to put a bat on the table is texture: the cave is alive, the abandoned tower is not actually abandoned, the shrine has been undisturbed long enough for nesting. None of these uses involve initiative.

If you do roll initiative for a bat, do it because the party blundered into a roost and a hundred bats erupted at once. Run that as environment rather than statblock. Have each PC make a DC 10 Dex save or be Surprised on the first round, then describe the air going dark and let them choose to fight or flee. The bats themselves do nothing useful. Their fly speed is 30 ft., they have no actions, and Blindsight 60 means they navigate the dark perfectly while the party flails. The threat is the wave, not the bite.

A bat's real combat use is as a Find Familiar option for a wizard NPC, where the bat scouts a corridor and reports back what it saw via Blindsight. If the party kills the bat, the wizard knows. That is the encounter. The bat itself is not the threat; the wizard at the other end of the link is.

Drop one bat into the room, do not roll for it, and let the players decide whether to be jumpy. The ones who flinch will tell you who is genuinely afraid of the dungeon.