You know what, I'm making both an omen and a prophecy because POINTS!
With the same meaning, but still...

Omen: There are two butterflies, one primarily white as snow and the other primarily black as midnight. Sunlight bathed them as they twirled about each other in perfect harmony. But, with a flash of light, they are gone, replaced by a different butterfly. This one had one white wing and one black wing, identical to those of the previous butterflies.
Suddenly, an incredibly strong wind blew the butterflies away, obviously killing them immediately. The world goes dark, as if the Sun was extinguished by the breeze.

Prophecy: When the wind blows the conjoined black and white butterflies away, the Sun will be extinguished.

Meaning: The black and white butterflies represent two apprentices by the names of Ebonypaw & Frostpaw (they are butterflies because they sometimes represent youth, and the cats are apprentices). Them being 'conjoined' into one butterfly is meant to describe their Squirrelflight and Leafpool-like connection. Ebonypaw and Frostpaw can feel each other's pain, emotions etc. The wind symbolizes BreezeClan, a name a group of rogue-like kittypets gave themselves to mimic the clans. They kill the apprentices when one is out hunting and stumbles onto their 'territory'. These deaths are meant to warn the clans of the fall of SunClan, represented by the 'world going dark, as if the Sun was extinguished'.

I used the omen in a fanfiction.