Walking around Sepermeru when entering an area it became completely dark, looking up I still saw the sun. There are also strange sounds and whispers in the place.
could be an event that failed to start or just ended and some audio and other effects lingered. Theres a few different events that happen in Sep. I know sometimes the demon summoning circle event (with the moon beast) in Unknown City, its darkness effect stays for what seems to be hours after the event ends.
Sep means september?
“Sep” is shorthand for Sepermaru.
Once you’ve been on the map for a few years, nearly every POI gets a shorthand or nickname.
Don’t tell them about “Sperm river”.
I went back there and there was no more darkness and strange sounds. But I was very curious to know what this event was. I was only able to do one in the nameless city and in Bocaneiro bay.
I please dont make me curious.
You see, you’ve done it your self too 
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Unnamed city.
Buccaneers bay.
Me for example, “warmakers dungeon” , never with the actual name of it.
And some thrall names, omg, i pronounce them randomly.
Too many difficult names 
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About sperm river? Heard it first time, but i guess it’s the noob river.