Traiborn's Key
The last key is with Wizard Traiborn, located on the second floor of the Wizard Tower, just south of Draynor Village.
Tell him that you need Sir Prysin's key, and then ask him if he has any keys around. He is totally confused.
He finally remembers a key, but he needs 25 bones to perform a ritual to open the box that it's in.
You can kill the wizards in the tower to collect bones, but a quicker way to obtain them would be to kill chickens. There's a farm east of Draynor Village (blue circle on the map).
You can give him the bones gradually. In other words, you don't have to give all 25 bones at once. However, it quicker to do it all at once if you are killing chickens for them. When you give him all the bones, a very short mini-cut scene will ensue where he performs the ritual and a wardrobe will rise from the floor. He will take his Silverlight Key out and give it to you.
Obtaining the Silverlight Sword
Once you have all 3 keys, go back to Sir Prysin. Another very small mini-cut scene will happen as he opens the box and gives the Silverlight 
to you, and which you then brandish about for a bit.
Hint: If you drop your Silverlight and talk to Sir Prysin, he will give you another sword. This way, you could have an extra in case you lose your sword after the quest, instead of having to buy one for 500gp later on.
Killing the Demon
Once you have all your combat equipment and food ready, head to the Stone Circle just south of Varrock. Once you get within close proximity of the circle, a big cut scene starts. You see three dark wizards and a wizard called Denath.
All the Dark Wizards begin to chant.
They finally summon the supremely evil Demon.
This might be the end for Varrock!
But you arrive just in time!
When you enter the circle, a wizard will start attacking you, so you will have to kill it and go to the Delrith. Or, you can wait until all the Dark Wizards are facing away from you and the Delrith is close to you and then run to him. With your Silverlight equipped, attack the Demon until he is in a weakened state. When he is in this state, pick the right incantation (refer to the incantation you wrote down earlier) and finish off the foul Demon.
Congratulations, quest complete!
Rewards
- You get to keep the Silverlight sword - If you ever lose it, you can buy another from Sir Prysin in Varrock Castle for 500gp
- 3 Quest Points