Quick! I need an omen!

[Mistah J]

Hey all, I need a wee bit of help here for my campaign,

An island from the Plane of Shadow has appeared in the ocean of the Material Plane.

It is not a big island (takes almost a day to walk across) but it is infused with darkness/negative energy/anti-magic.

The players are no where near it, but they ARE in the same ocean, on a ship.

I'm looking for something 'odd' they can see/encounter/interact with that will be a puzzling mystery for a while, then make sense when they encounter the island itself (or at least learn about it) later on.

Thanks for the help

[Ishpumalibu]

Well the shadow plane has moving landscape i believe, maybe landmarks they know are moved, or gone?

[Timebomb]

Part of the sky is permanently night (they can see the stars clearly through the patch), the effect acts like smoke and if the PCs follow the trail it leads to the island.

All life in the ocean they see traveling away form one direction (the island). Schools of fish, jellyfish, dolphins, merfolk, everything. If they look carefully they can see even a Kraken running away.
If there is anything that attracts PCs, it is something they should be running away from.

My 2cp

[Brambleman]

all the winds are blowing towards one location, from all directions. kinda hints thats where youre going too.

[Aerotan]

I'd start mild and crank up the 'weird' as they get closer, personally. Maybe on the outer edges, the sky goes briefly dimmer like a cloud passing over the sun, but is completely clear, or the water seems darker for a few moments, but clears up. Then as they get closer, twilight seems to set on them far earlier than usual, and lasts longer, and eventually there's an unscheduled eclipse out of nowhere, till finally the party sails up to an island that's completely shrouded in mist and darkness?

Stuff like that.