Disasters are special Fallen Angels, which every player can fight daily between 12:00 AM and 10:00 PM UTC (providing the disaster hasn't been defeated). Damage done by all players accumulates then resets every day.
You access Disasters through the Map or through the countdown in the lower left corner of the main screen.
Before battle, you can choose one out of 4 boosts available. One is a free revive, the other three depends on the world boss for that month.
It is generally recommended that players always choose the free revive unless a given boosts guarantees enough damage to go reach top 100 or more, or to claim a new reward from the Disaster Handbook.
Personal Ranking Daily Rewards
Guild Ranking Daily Rewards