The best replacement for nightmare is night terror.

The best alternatives to nightmare are rated as following:
★★★★★  night terror
★★★  dream
★★  dream eater
★  siegfried
★  mobzilla
★  astaroth
  hail to the king
  dream come true
  ender dragon
  toys
  night fury
  night terrors
  paladin eve
  vindicator
  red dead redemption
  nightmare
  seizure
  machariel
  laghodessa
  bad dream
  cephadrome
  blood manor
  dreams

Your alternative for nightmare is not listed? Suggest!
Speaking about nightmare, one probably shouldn't forget about red dead.

Other search terms related to nightmare and their ratings are:
★★★★★ red dead
★★★★ red dead redemption
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Google Trends

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nightmare
night terror
dream