Giant Scorpion

A giant scorpion on the party's side is unusual, and the unusual is the point. The most plausible framings are a druid's beast companion (some druids have weird taste), a desert ranger's trained mount or guardian, or a creature the party freed from a snake-cult pit and which has decided, in its small Beast brain, that they are family now. Pick one and stick with it.

In a fight the scorpion is a 52 HP, AC 15 wall with 40 ft. of speed and no named attacks in the stat block. Treat it as a positional piece. Park it next to the cleric to soak the first melee enemy that closes, or send it into the front rank as a blocker that enemies have to spend actions getting past. It will not flank, will not Help, and will not target the squishy back-line enemy you wanted dead. It hits whatever is in front of it.

Out of combat the Blindsight 60 ft. is the real value. The scorpion notices anything within 60 ft. that has substance, even in pitch dark or thick smoke. Use it as a tripwire when the party is camped in the desert and someone is taking the second watch. The scorpion does not understand language and will not signal an alarm cleverly, but it will start moving, and a player on watch who notices the scorpion stir gets a free Perception check at advantage.

Have the scorpion outlive the campaign arc that introduced it. A beast that was useful once and then became a pet that lives in the wagon for the rest of the campaign is the kind of detail that turns a party of adventurers into a household.

A giant scorpion is the desert's punishment for not watching where you sit. It has no listed actions or named abilities in this stat block, so the encounter rests on three facts: AC 15, 52 HP, and Blindsight 60 ft. The Blindsight is what makes it scary. Sand, dust storms, magical darkness, and Invisibility all fail to hide a creature within range, and players relying on Stealth or obscurement will discover this the hard way.

Use the terrain. A giant scorpion crawling out of dry ground at the party's feet, on initiative count 11, is a different scene from one shuffling toward them across open dunes. Bury it. Have the rogue trigger it by stepping on a brittle crust of sand, and let the rest of the party roll Dex saves to react before the scorpion's first turn. The 40 ft. walking speed and Beast intelligence (Int 1) means it will not flank or maneuver, but it will reach the squishy back-line PC in two rounds if nobody intervenes.

Combat is short. With AC 15 and 52 HP it absorbs about three rounds of focused fire from a level-3 party. The scorpion will not retreat. Beasts at this Wisdom score (9) do not back off. They fight until they drop. The choice is yours: let the party kill it cleanly, or have a second scorpion appear when the first is bloodied. Two scorpions on a level-3 party is a real fight. One is a speed bump.

Run them in pairs or with thematic dressing. A pair guarding a corpse means there is loot under the corpse. A scorpion that crawls out of an amphora the party just opened is a trap as much as a monster. Make the scorpion mean something for the scene, and the players will remember the moment more than the damage roll.

Telegraph the Blindsight in fiction. "It does not seem to look at you, but it always knows exactly where you are." That single line teaches the players what kind of creature they are dealing with.