Turael
The lowest-level Slayer Master, Turael resides in Burthorpe and poses you a unique Slayer Challenge. He has constructed a basement to house a variety of lethal beasts. To defeat the challenge, you must enter the basement and slay all the creatures in one visit. The challenge requires Level 3 Combat.
Mazchna
Found in north-eastern Canifis, the Slayer Master Mazchna tends to assign easy tasks. However, for his Slayer Challenge, Mazchna asks you to investigate the evil forces corrupting the Morytania fairy rings. To successfully face the challenge, you'll need Level 20 Slayer skill, Level 40 Combat and a completed Nature Spirit mission.
Vannaka
Slayer Master Vannaka can be a bit more demanding than others, requiring you to slay dragons. In order to pass Vannaka's Slayer Challenge, you'll need to bring him a scale from each of the four chromatic dragon types. Even scales from baby dragons count, so keep that in mind when choosing your battles. The challenge requires Level 40 Slayer skill, Level 55 Combat and completion of the What Lies Below mission.
Chaeldar
Somewhat of a mad scientist, Slayer Master Chaeldar created a unique fungicide spray that can counteract mutations occurring in the local zygomite population. Her challenge requires you to test the fungicide spray on mutated zygomites and then report back to her with the results. You'll need to have completed the Lost City mission and possess Level 70 Combat and Level 40 Slayer skill.
Sumona
Slayer Master Sumona despises the kalphites, so her challenge involves your fighting the Kalphite Queen. The task requires completion of the Smoking Kills mission and owning Level 85 Combat and Level 57 Slayer skill. Due to the challenge's difficulty, Sumona allows you to take a friend along to help. But in order to pass the challenge, you'll have to be the one who inflicts the majority of the damage on the Kalphite Queen.