Raven

A raven on the party's side is almost always somebody's familiar, somebody's omen, or somebody's mail courier. The stat block is intentionally tiny: 2 HP, AC 12, no actions, fly 50, Perception +3, passive Perception 13. It cannot fight. The reason it earns a tab on this site is that ravens carry more session-time than their CR 0 deserves, because GMs use them constantly and want to do it well.

Use the raven for vision and delivery. Fly 50 ft. with no listed actions means it is purely a tool for the player who controls it: scout the next room, drop a note on a windowsill, watch a meeting from a beam. Find Familiar gives a wizard or warlock the option to see through the raven's eyes as an action, which turns a 2-HP bird into a free reconnaissance loop. Don't let initiative get rolled with the raven on the field. The instant a hostile spell goes up, the raven flies for the rafters and waits.

Lean on the lore. A raven that has spent any time with a wizard tends to mimic words; the SRD entry doesn't grant Mimicry, but a typical raven absolutely repeats a phrase or two if you describe it. That is roleplay, not a stat-block ability. Players love a familiar that says "key", "key", "key" at the locked door, and they remember it for years. Just don't let the raven solve the puzzle for them.

Give the raven one specific behavior the player invented: it lands only on the left shoulder, it steals shiny coins, it clicks at every cat. The behavior becomes the bird, and the bird becomes a character.

A hostile raven is almost never a single bird. It is a witch's pet sent ahead to scout for the party, a corpse-eater on a battlefield the party is trying to search before nightfall, or, more often, the warning that a flock or a swarm is coming. The single-raven adversary entry mostly exists for the sake of completeness, since 2 HP and no attacks means a thrown rock kills it.

Run the raven as a problem the party cannot solve cleanly. It watches them from a tree, just inside its 50-ft. fly range, and reports back to whoever sent it. The witch knows exactly which way the party turned at the crossroads. The hag knows which PC is wearing the locket. Players will want to shoot the bird, and they should be allowed to, but every dead raven means a new raven will arrive within the hour. The pressure is the surveillance, not the combat.

If the party does kill the bird and try to read the corpse for clues, give them an omen. A raven sent by something Fey or Fiendish leaves a marker behind: a black feather that won't burn, a smear of ink-like blood, a single human tooth in its crop. None of that is on the stat block. All of it makes the encounter memorable.

Have the same raven appear three sessions in a row before anyone shoots it. The party should be the ones who decide it is hostile, not you.