Agony was a Lv. 3000 
Raid Boss from an event, fought at the 
Grand Colosseum. It used the 
Pain fruit and resurrected as a Tormented Boss at low health, going from 55,000 to 200,000 HP with attacks nearly 3 times faster.
How to get it
Event Boss (currently no longer spawnable): it appeared every 20 minutes at the Grand Colosseum, alternating with 
Ashen, announced by the message "Pain takes form at the 
Colosseum.". It despawned after 9 minutes. At low health, it resurrected as a Tormented Boss (200,000 HP).
What it drops
Summer TokenHow to Defeat It
The Colosseum is a PvP zone (forced on entry): fight it in a private server if possible. 
Buddha and long-range fruits work wonders there. Once tormented, its max damage is tripled and it attacks nearly 3 times faster - stay out of range and Instinct-trick its beams (Rage Release, Agony Nuke). 
Bomb's [V] (Self Destruct) deals enormous damage when it closes in on you.
Our take
Event Boss now unavailable: if a similar event returns, prepare a long-range fruit and a private server to avoid the PvP chaos of the Colosseum.
Update history
Raid Boss : Agony·Released : Update 27.5
Bosses
Agony
Ancient Beast
Awakened Ice Admiral
Beautiful Pirate
Bobby
Cake Prince
Cake Queen
Captain Elephant
Chef
Chief Warden
Core
Cursed Captain
Cursed Skeleton Boss
Cyborg
Darkbeard
Diamond
Don Swan
Dough King
Fajita
Fishman Lord
Gas Knight
Gorilla God
Greybeard
Heaven's Guardian