When in battle, and are losing drastically (basically when they think all hope is lost, and they might get captured, or even killed), Nightmare has a secret backup plan.
He can tell his gang to vent their life story out to him in their mind, therefore creating immense levels of negativity. Nightmare will suck up as much negativity he can, and his abilities and strength will double, or even triple, in power. This not only makes Dream weaker, but also makes Ink sad since he does get emotion (only positivity and negativity of course) from the Dreamtale twin's auras. This gives Nightmare a huge advantage. Therefore he could overpower them easily, without even much effort. But of course there are downsides to this, it could possibly turn Dream to stone for another 500 years, and his gang would have to go full on depresso-expresso mode to give him enough negativity, which he doesn't like.
This backup plan is only to be used in life-threatening, dangerous, situations, and only as a last-resort. Nightmare could cause some serious damage, mentally, and physically to other people, and even to himself if he's not careful.
- Bean