Seeker Synapse is a living island found in The Skull region of the Wushanko Isles. It is as described as "A living island (likely an evolved sea monster) with a brain at its centre and floating eyeballs as its sensory system." by the Archipelago map. Due to the name and description, it is likely a Seeker from Daemonheim, as opposed to a sea monster like the description states.
During The Brain mission of The Occultist's storyline, Zu Zu travels there in search of a brain for the ritual that would allow her to die.
Seeker Synapse only shows up after completing The Brain.