Gehenna

Gehenna was a prison island owned by Saron Deimos. It was reputable for housing particularly dangerous monsters and magical beings. Saron would regularly hand-pick prisoners who could be made loyal to him and handed them over to Zekaria, who'd recruit them into the Maniacs. Gehenna was eventually attacked and overrun by Finnick, who intended to use its obsidian-ward as a mask to operate an army in Aeygpt. Finnick was forced to retreat when Kanrik, Gaston, Rennata, Rilla and Florian investigated the prison. Zekaria was there too, having been sent by Saron to find out what was going on. Zekaria reported Finnick's indentity back to Saron, and Saron then became obsessed with capturing Finnick's power for himself. Since Finnick's attack, the island is a mostly deserted rock filled with Necora infection.