After you fail a nightmare test, an enemy is spawned and then your nightmare goes back to zero. So that is one way to get rid of nightmare.
There is at least one animus that benefits from having a high nightmare (the monster hunter). His level 3 ability gives a damage bonus equal to his nightmare.
Ah ha! I totally missed that sentence in the rulebook. I wasn't resetting nightmare after nightmare tests, resulting in kind of a slog. Thank you, can't wait to try again.