A little terror inspired by something I saw on a walk: the shadowhawk.
A shadowhawk is a vicious creature of the fey wilds. It builds its nest hanging from trees in forests with canopies thick enough to let little light through. Its nest is skull shaped with openings too small for the shadowhawk to pass through without its amorphous shadow form. When the shadowhawk perceives a threat to its nest, it leaks out through the eyes, nose, and mouth of the skull and launches into an attack.
Tiny fey, chaotic evil
Armor Class: 14
Hit Points: 9 (2d6+2)
Speed: 10 ft., fly 60 ft.
Strength: 6 (-2)
Dexterity: 16 (+3)
Constitution: 12 (+1)
Intelligence: 12 (+1)
Wisdom: 14 (+2)
Charisma: 13 (+1)
Saving Throws: Dex +5
Skills: Perception +5, Stealth +4 (+6 in dim light or darkness)
Senses: darkvision 60 ft., passive Perception 15
Challenge: 1 (200 XP)
Amorphous. The shadowhawk can move through a space as narrow as 1 inch wide without squeezing.
Shadow Stealth. While in dim light or darkness, the shadow can take the Hide action as a bonus action.
Sunlight Weakness. While in sunlight, the shadowhawk has disadvantage on attack rolls, ability checks, and saving throws.
Keen Sight. The shadowhawk has advantage on Wisdom (Perception) checks that rely on sight.
Draining Talons. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 2 (1d4) slashing damage and 9 (2d6+2) necrotic damage, and the target’s Strength score is reduced by 1d4. The target dies if this reduces its Strength to 0. Otherwise, the reduction lasts until the target finishes a short or long rest.
If a non-evil humanoid dies from this attack, a Shadow rises from the corpse 1d4 hours later.
If your players are venturing into dark woods, you might want to check out my low-level adventure module: The Cult of the Sleeping God.