Thanks to Raktavan and Draelan for this info
It seems that you need to finish sweet dreams quest first. (I haven’t got this quest). I believe you can get Nightfall without obtaining this quest though.
 Have a Lighthorn Monster, baby or teen.
Reward: 50 xp, 500 food
 Becky has a plan. Harvest 20 Crops!
Reward: 50 xp, 150 food
 Earn 200,000 coins.
Reward: 50 xp, 100 food
 Place 3 Fountains on the mountain to lure the mysterious monster out.
Reward: 50 xp, 340 coins
Note: Fountains cost 896,000 each.
 Hatch a Nightfall Monster!
Reward: 50 xp, 5 diamonds
Note: Nightfall is Plant/Shadow. 10 hour hatching time. Keep in mind that this is the same as Eeriewood.
 Evolve the Nightfall monster to Adulthood while Becky rests.
Reward: 500 food, 200 coins