Swarm of Ravens

A swarm of ravens as ally is the witch's familiar host or the corvid druid's wild shape companions. The Morrigan-style framing works cleanly: an old woman in a black shawl arrives at the camp, the ravens follow her, she introduces herself by another name and offers the party three scraps of advice in exchange for a name they would rather forget. The swarm is hers, and it lends itself to the party for as long as the favor lasts.

The tactical use is reconnaissance and intimidation. 50 ft. fly speed, Passive Perception 15, and a body the swarm can scatter and reform makes ravens the best long-distance scouting tool short of a Find Familiar. Send the swarm over the keep wall, have it count the guards, return with a feather dropped at the wizard's feet. In a fight, the swarm flies into the enemy spellcaster's space and forces concentration checks on whatever ongoing spell would have ruined the party's day. Damage resistance to Bludgeoning, Piercing, and Slashing means low-level enemies struggle to hurt it at all, while a single Magic Missile or Burning Hands will end the swarm in one cast.

The exit is the witch leaving. When the favor is done, she calls the swarm back with a single low whistle and walks out of camp without waiting for goodbyes. The ravens follow her into the trees. If the party tries to keep one bird as a souvenir, the bird will be gone from the cage in the morning, and the cage will have a single black feather inside it.

Have one of the ravens land on the gravest party member's shoulder and stay through a full scene of dialogue. The player will roleplay around it for the rest of the session.

A swarm of ravens is the omen made physical. The SRD 5.2 stat block lists no actions, traits, or Multiattack, which means the swarm you put on the table is a hazard with feathers, not a damage-dealing combatant. Run it that way and it works. Run it as a damage-dealer and it falls apart.

The opener is environmental. The swarm comes off a gallows tree, off a battlefield two days old, off the eaves of a haunted chapel. Passive Perception 15 means it notices the party at distance and decides whether to react. Use the swarm as a cloud that obscures vision and harasses casters trying to hold concentration. Rule that any creature inside the swarm's space takes Disadvantage on concentration saves and rolls a DC 10 Constitution check each round to maintain Concentration. That ruling is at-table and not in the stat block, so flag it openly to the players if you use it.

The damage profile relies on quantity and ruling. Without a listed Bite or Beak attack on the SRD 5.2 line, you can import the standard swarm chassis and rule the ravens deal around 5 to 7 piercing per round to a creature that ends its turn in their space. If you don't want to bolt on attacks, run it pure as a hazard. 50 ft. fly speed lets it intercept a courier, blot out moonlight on a roof, or shadow a guilty PC home for an evening. Resistance to Bludgeoning, Piercing, and Slashing means low-level enemies struggle to hurt it, but a single Burning Hands or a swung torch ends the fight in one cast.

Ravens scatter when half the flock is gone. Once Bloodied, the swarm breaks into individual birds and disperses to the rooftops, the trees, the open sky. None of them die. They are watching the party from three different rooftops by the next scene, and the party can feel it.