Gloom (CR 25)

Medium Monstrous Humanoid
Alignment: Usually lawful evil
Initiative: +26 (+18 Dex, +8 Superior Initiative); Senses: Blindsight 60 ft., Listen +35, and Spot +35
Languages: Common (probably)

AC: 40 (+18 Dex, +12 insight) touch 40, flat-footed 22
Hit Dice: 25d8+225 (337 hp); DR: 10/epic
Fort +17, Ref +32, Will +21
Speed: 30 ft.
Space: 5 ft. by 5 ft./5 ft.
Base Attack +25; Grapple +36
Attack: +10 keen dagger of human dread +54/+49/+44
Damage: Dagger 1d4+21/15-20 (+1d6 crit)
Special Attacks/Actions: Fear gaze, sneak attack +13d6
Abilities: Str 32, Dex 46, Con 29, Int 26, Wis 25, Cha 30
Special Qualities: opportunist, quiescence, spell-like abilities, SR 35
Feats: Cleave, Dodge, Improved Critical (dagger), Improved Initiative, Mobility, Power Attack, Spring Attack, Weapon Finesse (dagger), Weapon Focus (dagger); Epic Feats: Blinding Speed, Devastating Critical (dagger), Overwhelming Critical (dagger), Sneak Attack of Opportunity, Superior Initiative
Skills: Bluff +38, Climb +39, Hide +46, Intimidate +38, Jump +39, Listen +35, Move Silently +66, Spot +35, Survival +35, and Tumble +46
Advancement: 25+ HD (Medium-size)
Climate/Terrain: Any urban
Organization: Solitary
Treasure/Possessions: Nonstandard (just its dagger)

Source: Epic Level Handbook

Fear Gaze (Su): Viewing the gloom's face inspires terror. Creatures that meet gloom's gaze must succeed at a Will save (DC 32) or suffer the effects of a fear spell as cast by a 20th-level caster.

Opportunist (Ex): This ability functions like the rogue class feature of the same name.

Sneak Attack (Ex): A gloom is able to sneak attack as a 25th-level rogue, dealing 13d6 extra damage.

Quiescence (Su): A gloom is constantly it, as per the silence spell, although the area does not extend beyond the creature itself. This quality gives it a +20 racial bonus on Move Silently checks (it is still possible for a gloom to create noise, such as by bumping into something that scrapes on the floor or by sounding a chime). The gloom can lower this effect at will.

Spell-Like Abilities: At will - shadow walk; 3/day - true strike. Caster level 20th; save DC 20 + spell level.

A gloom has the uncanny ability to reach its prey by moving from shadow to shadow - literally. Combining that with its knack to remain totally quiet, the gloom often is able to attack and vanish before its prey's companions even realize something is wrong.