A timer will appear in the corner of your screen, counting down in 3 minuets. This is the way long you've got to stop the demon before he reaches Varrock. Run to the chaos change before your time runs out and you should see him. Once you spot him, the timer will dissapear. Before he sees you, proceed under the spell menu and then cast a new spell: Contain, with the runes that you recieved. When the spell hits Kacnia it will compact him into a small ball, very similar to what happens to you when you teleport. He will remain like this for 20 seconds. Once he's a ball, run him up and melee him. He won't have the ability to attack you. When he gets out of the ball, run back out of his melee range and throw contain again. Do this until he dies. Once he is dead, collect his bones (you can't bury then and they're known as"Kacnia bones) and come back to the fortune teller. Quest complete.
Reward. For beating this quest you receive the ability to cast the spell contain and accessibility to the pit where it is possible to runecraft space and time runes (with 65 runecrafting). You also get 2000 prayer experiance. What do you believe? Well for a start I will tell you that I've been granted permission by a mod to place this so dont tell me theres a sticky for this.
Check out https://www.rsgoldfast.com for more details.