At the 'teleport cost 0 anima' camps Atomos appears at night. He often times brings with him an assortment of mobs called deepvoid which are essentially gigantic version of various mobs in the game; ogres, zombies, arhiman, imps and skeletons etc. If caught off guard these mobs can murder you and should be either kited away or killed in a group. If you can get claim to these mobs they do drop overaspected crystals from what I hear and ina group they are not hard to kill, a light party can take one or two of them out (if you don't lag out from all the people at camps).
Atomos has pull-in, can occasionally move/run away, has nasty AOE damage moves and in general feels like a raid boss. think a full party of 8 with time and few adds can kill him, but he can despwn if you don't kill him quick enough and with so many people he is usually dead in 30-40 seconds anyways. He drops to the ground a thing called the voidstone which anyone (even those who didn't participate in the fight) can click and get an overaspected cluster. Its unique/untradeable and can be turned in at the foundation day NPCs for 3k grand company seals. Once you turn it in, you can get another one from Atomos. You can also click the voidstone more then once and get additional rewards like free anima (20 to up to 40 anima from what I hear), overaspected crystals, and buffs like flee which is essentially chocobo level speed on a 30 minute timer that does NOT go away upon damage or engaging another mob. It goes away when you die though. Some people I hear last night were using the buff on Garuda or and other fights to run past mobs or dodge specific attacks.
The other camps Atomos spawns at are at places like Camp Horizon, Black Brush and several other new camps (Dragonhead) these happen during the day time and Atomos cannot be targeted and thus can't be killed. Stands to reason that at some point in the future he will be selectable later on and the camps will be teleport cost free if the other camps are anything to go by. Would be nice by the time we get the 'end of the world' event that all aether crystals are anima free.