I have found this fortress in Deep Dark. Have a lot of Spawners, and chests. Can i take the spawners with diamond dolly and put in my islands and loot the chests?
As DirtyDave30 says, they are part of the world-generation of the deep dark. As long as there are no obvious player-alterations to these, or claims if they are enabled, then they are free for all to take. They are similar to dungeons from various dungeon-mods, which add dungeons/temples/etcetera to the world.
all the mob spawners should be worldgen and claims are not enabled in the deep dark, so ya i think its safe to take them