Clarifying on the above post from Ouch!, 4x Over-aspected Crystals can be redeemed at an NPC for 1,000 grand company seals (no anima). Beating Atomos will cause a Voidstone to appear on the ground. I usually am able to get four sets of 'rewards' from the stone by examining it repeatedly until it's gone. The rewards are at random from the following...

5x Over-aspected Cyrstals
7x Over-aspected Cyrstals
7x Over-aspected Cyrstals (and you get moved back a few yards / screen goes black temporarily)
20x Anima
40x Anima
1x Quickened status effect
1x Heavy status effect

You can also get an Over-aspected Cluster the first time you beat Atomos which can be redeemed at an NPC for 3,000 seals.

Atomos spawns at 18:00 and 00:00 (game-time) at various orange aetheryte camps where he is defeatable. He spawns at 06:00 and 12:00 at various blue aetheryte camps, where is not able to be attacked. When he spawns, various other high level aggressive enemies also spawn. On occasion, Atomos will move away from the camp after spawning. Atomos disappears (with no voidstone left behind if he was not killed) a little over three game-hours after he spawns.

On Sargatanas server, you can expect to find many people ready to kill him with you at Camp Bluefog. Upon getting voidstone rewards, people then travel to Camp Brittlebark for another Atomos kill (keep in mind he appears at all the orange camps, so multiple kills are possible) before Atomos disappears. Please keep in mind that when you teleport, you will likely have to wait 1-2 minutes to reach your destination due to the vast number of people teleporting at once. You should still make it in time to beat Atomos at Brittlebark, though.

It is not known how long Atomos will be around, but it is suspected he will be there until the current servers go down in early November in preparation for A Realm Reborn.

It is not known how long the grand company NPCs where you turn in your cluster/crystals will be around. As they are around as part of a seasonal event, it is suspected that they will at some point before the current servers go down in early November in preparation for A Real Reborn.

You do not need to attack Atomos in order to examine the Voidstone, however if you do not try to attack Atomos and instead just wait around for the voidstone while other people kill Atomos then it should be noted that you're a terrible person.