Necrotaphira island is a very advanced island/city in the new world, here, technology is the greatest and is comparable to vegapunk's.
Lixis D. Korosume has offered them his protection in return for a few things.
Necrotaphira Island/City is a very dark looking city filled with geniuses and masterminds. The buildings are very tall and have the strangest shapes. Nobody knows what is inside of those building except for Lixis D. Korosume and the workers.