They come out of the ground in groups and when exposed to bulldog light they catch fire and burrow back into the ground.
Sometimes they can be damaged if they burrow into a protruding boulder.
It seems like they spawn more nameless whenever they unearth themselves. They are easiest when you have enough dps to kill them before they burrow. Attack them with friends if possible.
If you plan on building in the bio luminescent area be sure to have plant z on the outside of your base. Preferably out of reach of the nameless. Growing these will keep them away from your base, thus preventing a reaper king from knocking on your front door.