Appearances in canon
The first mention of the island of sanctuary was in the 14th volume of the manga, when Dr. Balkus made his decision to tell Purg'stall and Shin about the island, and the fact that Alkanphel had been forced to return there due to his "sleeping sickness".
Silha is a tropical island in the North Atlantic, home to the experimental creatures that the Advents left behind when they vacated Earth.
Mermaids, Fauns, Dinosaurs, various other prehistoric creatures, and at least one Fairy, make up the fauna of the island. The flora, in contrast, seems to be that which is typical of tropical islands. There is also one humanoid wolf - possibly more - that lives on the island as well, though what it hunts is a matter for some speculation.
There is also a temple on the island, one that was built for Alkanphel after he fell back to Earth in the aftermath of the energy-drain caused by his destruction of the large meteor that the Advents attempted to use to destroy the planet.
Appearances in fanfiction
Son of Mine ('verse)
Near the end of the second story, Alkanphel was forced to return to his sanctuary island, leaving Imakarum and Ingriam to cope with his absence in their own ways. In the third story, he stays there for almost the full duration of the tale, and only returns to make an announcement to the Council as a whole.
At the beginning of the fourth story, Alkanphel awakens from his slumber to give some very specific orders to the members of his Council. During the beginning of the fifth story, Imakarum is left to reflect on Ingriam's presence on the island, as well as the reason for it.