Harbinger of doom
A sign, warning of bad things to come.
We now use 'harbinger' in a metaphorical sense, meaning 'forerunner; announcer'. With that meaning, almost anything can be harbingered - not exactly an everyday word that, but one that has been in occasional use since the 17th century. We sometimes hear of 'harbingers of Spring', or 'harbingers of day', but it is the 'harbingers of doom' that are the busiest in our present-day language.
The original meaning of harbinger was quite specific and had nothing to do with any of the above. In the 12th century, a harbinger was a lodging-house keeper. The word derives from 'harbourer' or, as they spelled it then, 'herberer' or 'herberger' , i.e. one who harbours people for the night. 'Herberer' derives from the French word for 'inn' - 'auberge'. 'Ye herbergers' are referred to (as common lodging-house keepers) in the Old English text The Lambeth Homilies, circa 1175.
By the 13th century, 'harbinger' had migrated from its original meaning of lodging keeper, to refer to a scout who went ahead of a military force or royal court to book lodgings for the oncoming horde. This is the source of the 'advance messenger' meaning that we understand now. Geoffrey Chaucer was the first to record this meaning of 'harbinger', in The Man of Law's Tale, circa 1386:
The fame anon thurgh toun is born
How Alla kyng shal comen on pilgrymage,
By herbergeours that wenten hym biforn
[The news through all the town was carried,
How King Alla would come on pilgrimage,
By harbingers that went before him]
It was some centuries until the figurative usage, when people began to speak of harbingers of things other than approaching royalty or house guests. The first suggestion of 'doom', or 'ruin' as the Edinburgh Advertiser had it, came in September 1772:
"The spirit of migration [from Scotland] is the infant harbinger of devoted ruin."
It is rather appropriate to find that early usage coming from Scotland. The character of Private Frazer, in Dad's Army, a well-known (in the UK at least) BBC television series, was based on the perceived gloomy attitude of his race. John Laurie, who played the lugubrious Frazer, was the archetypal stage Scotsman and the show's line "We're all doomed, doomed I tell ye" became something of a catchphrase for him.
Those pessimistic harbingers of doom who first decided that 'the end of the world is nigh' lived in the 19th century. The earliest printed example of that phrase that I have found is from James Emerson Tennent's Letters from the Aegean, 1829:
"Achmet, our janissary, calculating from the decay of their empire and the daily fulfilment of the predictions of Mahomet with regard to the final resurrection, have come to a conclusion that the end of the world is nigh at hand."