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.
Make sure if you have a blue base that you have lots of light in it otherwise they will spawn in your base :/...to kill them all you need is a light pet and something strong like a drake, crab, reaper, spino, mega ect. Make sure your light pet is turned off so they spawn and then run around making more spawn then turn your light pet on and go ham...this is an effective way to get nameless venom as light will nerf them a lot and stop them from spawning :) enjoy.