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

• Lexulous
• Wordscraper
• Wordosaur

v. To attack (someone) unfairly, especially in an underhand, deceitful manner: "Some backstab each other and threaten to settle their differences with a punch” ( Thomas Boswell).

v. to attack someone (especially verbally) unfairly in a deceitful, underhand, or treacherous manner, especially when they're not present in the place or situation that it happens. (as if stabbing them in the back). See backbite.

"Greens are immune to poison and have a "backstab" ability in combat, whereas blues can swim and are the only ones who can damage magical enemies."

backstabber