Nightfall | Eclipse is a great card that helps counter Darkness's main weakness: low attack. It gives +1/+2 attack and +1 health to all Darkness and Death creatures. The permanent does not stack, so playing two Nightfalls will not give creatures +2/+2. An unupgraded and upgraded version on the same field will not stack as well. Note that it gives creatures on your opponent's side of the field the buff as well.
Nightfall | Eclipse works great with any deck that primarily focuses on Darkness or Death creatures.