Fallible Invisibility. The skulk is invisible. This invisibility can be circumvented by three things:
Trackless. The skulk leaves no tracks to indicate where it has been or where it's headed.
Claw. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit:6 (1d4 + 4) slashing damage plus 3 (1d6) necrotic damage.
Some children have imaginary friends that their parents can't see. Sometimes those invisible friends aren't imaginary.