Gutter Wraith
Traits
Sunlight Sensitivity. In sunlight, the Gutter Wraith has Disadvantage on attack rolls and Wisdom (Perception) checks. It also cannot use Alley Step or Ambush Predator while in sunlight.
Alley Step. As a Bonus Action, the Gutter Wraith teleports up to 30 feet to a point it can see that is in Dim Light or Darkness (a sewer grate, a doorway, a shadow). It can then take the Hide action.
Ambush Predator. The first attack the Gutter Wraith makes while hidden deals an extra 14 (4d6) Necrotic damage.
Incorporeal Slip. The Gutter Wraith can pass through any gap no narrower than 1 inch (a grate, a crack, a keyhole). This movement doesn't provoke Opportunity Attacks.
Actions
Multiattack. Makes two Rusted Shiv attacks.
Rusted Shiv. Melee Attack Roll: +6, reach 5 ft. Hit: 11 (2d6 + 4) Piercing damage plus 7 (2d6) Necrotic damage. If the Gutter Wraith is hidden, plus an extra 14 (4d6) Necrotic (Ambush Predator).
Life Sip (1/Turn). When the Gutter Wraith deals Necrotic damage, it gains HP equal to half the damage dealt (can't exceed its HP maximum).
Bonus Actions
Alley Step. (See Traits above.)
Reactions
Fade to Grate. When the Gutter Wraith takes damage while in Dim Light/Darkness: halves the damage and moves 30 feet (doesn't provoke Opportunity Attacks).
CR Rationale: Defensive — AC 14 / HP 79, right above the CR 4 target; resistance to nonmagical weapons raises its effective HP to ~120 for low-level parties, but Fade to Grate depends on darkness and shuts down completely in sunlight. Offensive — an opening turn deals 2 × (11 + 7) + 14 = 50 damage, notably above the CR 4 band (~30/turn); subsequent turns drop to 36. Burst exceeds the average, average stays within the band — the Lurker profile.
How to Use
The city's night shift. It emerges from the sewers, catches its prey in shadow, deals 50 damage in one turn, and vanishes down a grate.
Best use: a chase. The party thinks they've cornered it in a corner — it slips through a crack with Incorporeal Slip. They catch it on a street — it climbs to a rooftop with Alley Step. This is the monster that teaches low-level parties "killing the enemy isn't enough — you have to catch it first."
Use two Gutter Wraiths in sequence: one hits and runs, and while the party chases it, the second ambushes from behind.
Key: Bright Light. Every part of its mechanism depends on darkness: Alley Step (needs Dim/Dark), Fade to Grate (same), Ambush Predator (needs to be hidden), and Sunlight Sensitivity shuts all of it off at once in sunlight.
Light, Daylight, a ring of torches, or even postponing the fight until dawn. In a lit plaza, the Gutter Wraith is: AC 14, HP 79, attacks with Disadvantage, no teleport, no escape. An ordinary target. In a dark alley, it's a nightmare.
This isn't a fight — it's a lighting puzzle.