Hive Annihilation becomes available when you reach a certain level and progressed far enough in the main quest.
Tap the Activity Overview icon on the top right, and you can enter Hive Annihilation from the Get Stronger tab as illustrated below:

Hive Annihilation levels, which are selected from the interface above, are solo only.
Completed levels can be viewed but cannot be attempted again.
You can attempt the current Hive Annihilation level as many time as you want.


There is a countdown timer upon entering Hive Annihilation, after which the boss will spawn. Then Two minutes after that, assuming the boss is still alive, it will become enraged making it harder to deal with. You may check the timer any time on the left of the mini-map.

The rewards for completing levels are sent to your mailbox.

The icon on the bottom right of the page will bring you to the Hive Annihilation Leaderboard. There you can check the regional or global rankings of players. The ranks are based primarily on levels, and if two players have cleared the exact same level, whoever cleared it earlier are be placed higher.