Cinder Harrier
Traits
Fire Form. The Cinder Harrier can pass through any gap no narrower than 1 inch. It can enter a creature's space and stop there. If it ends its turn in an enemy's space, that enemy makes a DC 17 Dexterity save; Failure: 10 (3d6) Fire damage.
Blazing Wake. Every square the Cinder Harrier passes through this turn catches fire until the start of its next turn. A creature entering these squares or ending its turn there makes a DC 17 Dexterity save; Failure: 10 (3d6) Fire damage.
Searing Momentum. If the Cinder Harrier moved at least 40 feet this turn, its attacks deal an extra 14 (4d6) Fire damage.
Untouchable. The Cinder Harrier gets a free Disengage on its turn. Also, Opportunity Attacks against it are made with Disadvantage.
Heat Haze. Ranged attacks against the Cinder Harrier from more than 30 feet away are made with Disadvantage (rising heat shimmer).
Actions
Multiattack. Makes three Ember Lash attacks.
Ember Lash. Melee Attack Roll: +10, reach 10 ft. Hit: 13 (2d8 + 4) Fire damage plus 7 (2d6) Slashing damage. If the Cinder Harrier moved 40 feet this turn, one of its attacks gains an extra 14 (4d6) Fire (Searing Momentum).
Cinder Storm (Recharge 5–6). The Cinder Harrier moves its Fly Speed (doesn't provoke Opportunity Attacks), leaving a whirlwind of ash behind it. Every enemy within 10 feet of its movement line makes a DC 17 Dexterity save.
- Failure: 35 (10d6) Fire damage, and the target has the Blinded condition until the end of its next turn (ash fills its eyes).
- Success: Half damage, no blindness.
Bonus Actions
Ashen Veil. The Cinder Harrier turns into a cloud of smoke and moves 40 feet (doesn't provoke Opportunity Attacks) and takes the Hide action. While in smoke or ash, it rolls Stealth checks with +14.
Reactions
Flare. When the Cinder Harrier takes damage: the attacker makes a DC 17 Dexterity save. Failure: 10 (3d6) Fire damage. The Cinder Harrier then flies 20 feet (doesn't provoke Opportunity Attacks).
CR Rationale: Defensive — AC 16 / HP 162, right above the CR 9 target; Fire immunity plus nonmagical weapon resistance raises its effective HP, but Cold Vulnerability alone balances it out (Cold damage is doubled — 162 HP becomes an effective 81 against Cold). Heat Haze and Untouchable make it hard to hit. Offensive — 3 × 20 + 14 = 74 damage per turn, above the CR 9 band (~60/turn); without Searing Momentum it drops to 60, meaning its calibration depends on movement. Cinder Storm plus Blinded is a strong one-time position breaker. Save DC 17 matches the CR 9 norm.
How to Use
The fastest thing on the lava fields. The Cinder Harrier isn't a tower, it's a whip: it flies 80 feet, strikes three times, flies off, leaving a burning trail behind it.
The real danger is the combination of Blazing Wake and Cinder Storm. In a volcanic arena, the Cinder Harrier turns half the map into burning squares within a few turns. The party looks for "somewhere safe" — and there's less and less of it.
Ashen Veil makes it doubly infuriating: it doesn't just flee, it disappears. Stealth +14 in smoke, and inside that smoke it sees via Blindsight — the party can't see while it can.
Arena suggestion: an ash-raining caldera, rivers of lava, narrow stone bridges. Burning squares push players off the bridge.
Key: Cold. This is a weakness that leaves no ambiguity: Vulnerability. Cold damage is doubled. A 162-HP monster becomes an 81-HP monster against Cold.
Ray of Frost, Cone of Cold, Ice Knife, Sleet Storm, a +1 Frost Brand, even swapping a Ranger's Hail of Thorns for a Cold cantrip — a single character with Cold damage ends this fight in half the time.
Secondary key: Slow it down. Searing Momentum depends on the 40-foot movement requirement. Slow, Web (Restrained is immune, but the ground slows it), a narrow tunnel — damage drops from 74 to 60. But the real solution is ice.
A party with no Cold damage shouldn't enter this fight. This is the lesson the monster teaches: damage-type variety isn't a luxury, it's a necessity.