Zamorak is the main villain in SFCII. He assasinated Guthix, and expected nothing to go wrong in his plot to take over RuneScape afterwards (with Saradomin also dead by taking the Spontaneous Combustion Pill in SFCI). However, that's when you and the C journeyed to Hell to stop him.
Zamorak was protected by his five Demon Bodyguards, which acted as a boundary for you and the C during SFCII.
The being himself is amazingly powerful. You killed him as a god of balance.
-At the beginning of SFCII, Zamorak was the second-strongest being in RuneScape, behind Guthix. If he didn't have the opportunity to assassinate It, Guthix could have easily defeated him.
-He is the second god battled by You in the CYOA, the first being Guthix, although this was during training.