Threatening
a. & n. from Threaten, v. -- Threat"en*ing*ly, adv. Threatening letters (Law), letters containing threats, especially those designed to extort money, or to obtain other property, by menaces; blackmailing letters.
Shades of meaning
Fernald has no entry of its own for threatening; he lists it under imminent.
Where he writes about it:
Imminent is more immediate, impending more remote, threatening more contingent. — from imminent
An impending evil is almost sure to happen at some uncertain time, perhaps very near; an imminent peril is one liable to befall very speedily; a threatening peril may be near or remote, but always with hope that it may be averted. — from imminent