Darkness

Darkness is the absence of light, creating an area where most creatures can't see.

An area of Darkness is Heavily Obscured.
A creature trying to see something in Darkness effectively has the Blinded condition with respect to whatever the Darkness conceals.
A creature with Darkvision treats Darkness within that sense's range as Dim Light, letting it see there in shades of gray (no color).
Attacking a target you can't see imposes Disadvantage on the attack roll; a target you can't see has Advantage on attacks against you.
While Blinded, a creature can't benefit from anything that requires it to see its target.