Write a murder mystery like Umineko about a group of friends that go to their mansion in their private island, Rokkenjima, to hang out, but they are stuck for two days due to a typhoon.
Then a mysterious letter with the wax seal of a one-winged eagle appears at the dinner table, from an unknown witch called Beatrice, that claims she is going to kill them before the typhoon passes.
Then they are mysteriously murdered at random one by one. One of them is the culprit, that claims to be the mysterious witch, as only they are on the island. And as perfect examination of corpses aren't available, even the victims could be the culprit. And as only nine people are on the island, it has to be one of them.
Furthermore, victims all die under seemingly impossible circumstances, within locked rooms.
The story ends with all of them dead. The last sentence in the story is: "When the seagulls cried, none were left alive."

The characters should be introduced and their personality summarized in the story, then described as dead randomly.

The characters are:
Arian, owner of the island, Anahita, Arian's wife
Mobin, Ali, Amin, an Iranian nationalist, Saber, Mobin's old rival, MohammadMehdi, an anti-nationalist and Amin's rival, MohammadHossein, Amir, Jason, Ramtin

The culprit should be left open to interpretation.

Don't sort it by chapter.