Raids come in many forms. Typically raids are when one, or more, special monsters appear. You can fight these monsters using BP and they do not fight back. You cannot call your friend's monster in assistance. Rewards are given typically based on the performance in the raid.
Private Raid[]
Private Raids, or Tower Raids, are raids which a monster appears during battling monsters in the regular towers. These appears appear single and have varying stats based on which tower you found them. Certain monsters appear only in certain towers. Sometimes a raid monster may give out a mix recipe upon completion. Here is a list where you can find the Private Raid Bosses
Mix Recipe[]
Mix Recipes can be found under the mix tab. Mix is combining two tokens to achieve a different token. This is the only way to receive certain tokens such as Duelist or Pirate. For a complete list please visit the mix recipe page, along with a list of the possible raids and their locations.
World Raid[]
A world raid is a scripted event which occurs during set time intervals. World raids are like friend raids, except four extremely strong monsters are summoned. Team up with players around the world to take down these beasts - 3 of which will have some weakness for bonus damage - and receive rewards based on your damage ranking. These rewards may include tokens and essence and for top rankers, the monster token for that raid. For a full list of World Raid monsters visit the World Raid page.
The times for the World Raid are (in -5 hours EST):
Monday: 12pm
Tuesday - Friday: 12pm, 5:30pm-9:30pm, and 11:30pm
Weekends are random.
Costume Raid[]
The costume raid is a special world raid that occurs on Mondays. The costume raid tyically has much stronger monsters. For the top 1% they will receive Tiana's costume for that raid. Her costumes can be changed and viewed in her closet.