Attain Pure White World Tendency in World 5 -- Valley Of Defilement
Go to 5-1. Continue through the map until you reach the rats at the bottom of a ramp.
Kill the rats and across the way you will see a Giant Depraved One. You do not have to kill him, but it is recommended.
About halfway across the ledge connecting the 2 areas there will be a ladder to your left. Climb up this ladder and you will proceed across several ledges and kill several Depraved Ones, including a Giant Depraved One as you cross a bridge.
Climb up to a dead-end cave and you will see three Giant Depraved Ones.
Dispatch these three, and the Istarelle will be yours for the taking.
It is shorter to get to the ladder coming backwards from the 5-2 Archstone, but you will have to get past two Giant Depraved Ones from that direction.
Copy/pasted from here:
http://demonssouls.wikidot.com/istarelleVideo clip of me running from 5-2 back to the weapon.