Scree Dancer
Traits
Sure-Footed. Stone, rubble, ice, and steep slopes are not Difficult Terrain for the Scree Dancer. It also climbs vertical surfaces without using its hands and takes no fall damage (up to 100 feet).
Nimble Escape. Gets a free Disengage on its turn (no action required).
Momentum. If the Scree Dancer moved at least 30 feet this turn, its attacks deal an extra 10 (3d6) Bludgeoning damage.
Rockslide Wake. Squares the Scree Dancer moves through become Difficult Terrain until the start of its next turn (falling rocks).
Actions
Multiattack. Makes two Shale Claw attacks.
Shale Claw. Melee Attack Roll: +9, reach 5 ft. Hit: 15 (2d8 + 6) Slashing damage. If the Scree Dancer moved 30 feet this turn, plus an extra 10 (3d6) Bludgeoning (Momentum).
Scree Burst (Recharge 5–6). The Scree Dancer moves its Speed (doesn't provoke Opportunity Attacks) and collapses the slope beneath its feet. Every enemy that ends within 10 feet of its movement makes a DC 16 Dexterity save. Failure: 22 (4d10) Bludgeoning damage and Prone. Success: Half damage, no Prone.
Bonus Actions
Cliff Leap. Jumps up to 30 feet (horizontally or vertically). This movement doesn't provoke Opportunity Attacks.
Reactions
Slip Away. When an enemy hits the Scree Dancer with a melee attack: the Scree Dancer moves 20 feet (doesn't provoke Opportunity Attacks).
CR Rationale: Defensive — AC 16 / HP 134, right above the CR 7 target. Offensive — 2 × (15 + 10) = 50 damage per turn, dead center of the CR 7 damage band (~50/turn); without Momentum it drops to 30, meaning its calibration depends on moving — which is exactly the role's contract. Scree Burst provides burst value but is limited. Save DC 16 matches the CR 7 norm.
How to Use
In a mountain pass, at a cliff's edge, on a narrow trail. The Scree Dancer never stays in one place for two turns. It hits, retreats with Slip Away, then covers 50 feet next turn and hits again — the party can never pin it down.
Rockslide Wake leaves a trail of Difficult Terrain behind it: the more the party tries to follow, the slower they get, while it speeds up. Two Scree Dancers can keep a party occupied on a mountainside for hours.
Key: Stop it. All of Momentum's damage depends on the movement requirement. Grappled, Restrained, Speed 0 — whichever one, its damage is halved. Web, Hold Person (not an Elemental, but it has the Fey tag — DM's call), a Grapple build, Spike Growth, or simply squeezing it into a room, cave, or narrow crevice. Fight it on an open slope and you lose; pull it into a confined space.