Sarixs is located at the entrance at the Slayerdungeon of Prifddinas. If your Slayer level is 95+ and your combat level is 125+, you can ask him to fight him. If you win the fight, you will be rewarded with 220,000 Slayer exp for the first time and 22,000 for the second and any further win (you may kill him once a day). Note: Sarixs will wait until you attack him. So take this chance you use any potions, etc...