ok i have just done desert treasure =) thanks to runecrypt =] and i want the ghostly robes, i got the top. and the mysterious ghost told me to go to find rennard and were is he?

Here's the complete list. I think you need to do it all in order.

Rennard
The Crashed Pirate Ship in Level 56 Wilderness.
Bandit Camp in the Wilderness.
Bandit Camp Bar in the Desert.

Kharrim
The Wilderness Graveyard (not the Cemetery).
Prayer Altar in Level 44 Wilderness.
End of the Lava Maze near the Lesser Demons.

Lenissa
In Port Sarim, near the boat from the Dragon Slayer Quest.
Upstairs of the Falador Party Hall.
The first floor of the Wizard's Tower, near the Bookshelf.
Inside the Entrana church building.
Near the Gnomeball Court

Dhalak
Upstairs in the Prayer Guild, near the Altar.
Inside the Falador Party Hall.
First Floor of the Wizard's Tower near the Bookshelf.

Viggora
Paddewa, Edgeville Dungeon, near the Earth Warriors.
Second floor of the Rogue's Castle in the deep Wilderness.
Second floor of the Canifis Slayer Tower, just before the Infernal Mages.

That's about it I think.

Each person has different ways.
So just keep trying the ones bling told you, and it may be all at random.
But in the end you will have.

Just a tip, once you find him, talk to him to get the part of the robe, then drop the piece and get more, unless you only want one set

More bank space ftw?