erm your supposed to kill Elvarg which is lvl 80+, close to a lesser demon which is also about the same level
In melzars maze you have to kill a lesser demon to open a door for a map piece.
A quest walkthrough for Demon Slayer can be found here: www.runescape.wikia.com/wiki/Demon Slayer
In the maze for Dragon Slayer? well anyways it is in the chest after you kill the lvl 82 lesser demon!
you need at least a mithril armour to prevent dark wizards to attack you. With silverlight , you can kill it by 1 shot. (silverlight is weakness to any demon , including the demon you have to kill in mezler maze when you do dragon slayer.)
look on runehq
After you have completed the 'demon slayer' quest you get it as a reward. >.<
If this is part of the Demon Slayer quest, then the link below should direct you to your answer.
By doing Demon slayer quest
Talk to Gypsi Aris and she will give you it.
The Sword you used to kill the demon with, the sword is good against demons.
It is different for everyone. You have to go and write it down.
Finish the quest "Demon Slayer
bucket of water