This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

n. Agent noun of frighten; one who frightens.

"Although for much of the time the various teams within the city operated in tandem, respecting each other's territory, there were sporadic turf wars characterised by tit-for-tat shootings, with kneecappings a popular form of warning or 'frightener'."

frightened