“Atheon, Time's Conflux” is a Command unit of the Vex. It has extraordinary abilities within its own realm.
About 20 seconds after Atheon, Time's Conflux has arrived and been activated, it will send 3 random guardians through a time portal. Just watch for the message saying that Atheon has "opened the time stream".
Outside the time stream the rest of the team still has a task to do. They must coordinate with the team and open the portal that they were sent through. They will also need to defend themselves against the Supplicants that Atheon has called forth.
When the Portal team re-emerges from it, this will trigger the buff Time's Vengeance. The team should focus all fire on Atheon as it will now take up to 4 times more damage while the Guardians enjoy vastly decreased cooldowns on their grenades and supers. Focus all the fire and damage for these 30 seconds on the Raid Boss.
Guardians tend to approach this battle one of two ways. The first, they all fire on Atheon. This spread the aggro out but leaves the team more vulnerable from damage. The shield bearer should be using the super as much as possible because of its high damage.
Atheon will become enraged about 8 minutes of battle. There is a one minute warning before that happens. While enraged it will attack far more frequently. It will also summon in a very significant increase of Supplicants.
Hard Mode Change
While facing Atheon in hard mode, it will also detain Guardians. Much like the Templar, these bubbles will kill any Guardian that leaves them. They must be destroyed. It will be done in conjunction with sending the other half of the team through a time stream.