Cerberus

A 91 Slayer boss with 600 hitpoints that changes twice as its health falls — three souls, then lava.

Cerberus is a Slayer-locked boss: you need 91 Slayer to attack it, and it pays 690 Slayer XP a kill. It sits at combat level 318 with 600 hitpoints and a max hit of 23, and the fight changes twice as its health drops.

Requirements

The three souls

Below 400 hitpoints Cerberus starts summoning souls, and from then on it does so every seventh attack. There are three, and each one attacks with a different style — magic, ranged and melee — so the overhead prayer you need depends on which soul rose, not on Cerberus itself.

The lava phase

Below 200 hitpoints Cerberus starts deploying lava pools every fifth attack — three at a time. Landing in one costs 10 damage, and standing in it costs 15 a tick after that. The last third of the fight is spent moving, not attacking.

Drop table

Taken straight from Ashmarch's own drop data. Rates are per kill on Regular mode with no drop-rate bonus.