You can kill the wizards in the tower to collect bones

Traiborn's Key: The last key is with LOCO Gold 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 Silver-light Key out and give it to you.

Take the red path to a small room, and the key should be on the ground, surrounded by corpses. Just click on it and you'll get a rusty Silver light Key.