Three kings in one room, three different styles, and four rings at 1/257 each.
The three Dagannoth Kings share a room under Waterbirth Island. Each is combat 303 with 255 hitpoints, and each can only be hurt by one combat style: the other two do no damage at all.
Getting there
Detail
Value
Location
Waterbirth Island dungeon, off Rellekka
Teleport
Teleport menu > Bosses > Dagannoth Kings (30,000 coins)
Boat
Rellekka to Waterbirth Island, 1,000 coins
Requirements
None. Bring gear for the king you want to kill
Other routes
Boss teleport token, Master donator ring
Mechanics
King
Attacks with (max hit)
Hurt only by
Pray
Dagannoth Supreme
Ranged (38)
Melee
Protect from Missiles
Dagannoth Prime
Magic (67)
Ranged
Protect from Magic
Dagannoth Rex
Melee (36)
Magic
Protect from Melee
They share a room, so all three can attack you at once. Killing just the king you are equipped for is a fine trip. Each king counts toward a dagannoth Slayer task.
Rewards
Unique
From
Chance
Archers ring, Seercull
Supreme
1/257 each
Seers ring
Prime
1/257
Berserker ring, Warrior ring
Rex
1/257 each
Pet (one per king)
Each king
1/1,800
Drop table
Taken straight from Ashmarch's own drop data. Rates are per kill on Regular mode with no drop-rate bonus.