About 350 hours into the game and I have no idea what Ash Lake is.
There's a tree on the opposite side of blighttown that has a couple false walls, go through there, through the great hollow and boom, ash lake, home if the black hydra and dragonbros.
Great hollow...? I've heard of it but never seen it. It is in blighttown huh? Is it worth checking out for fun? Ah heck, I'm going whether it is fun or not.
Depends on your definition of fun. It's opposite the boss fight with Quelaag, there's a very large tree with a pretty obvious ramp going up it, also a spawn for invaders. Break a wall, there's a treasure chest, break the wall behind it and you're in the great hollow. It's full of narrow branches, deadly falls, curse frogs, and as soon as you get to the bottom, you enter ash lake. You are greeted by the Black Hydra, Darkroot Hydra's big brother. Use the same tactic, but it's faster, stronger, more aggressive, and can follow you throughout the area.
EDIT: Forgot to mention that the great hollow is home to at least 10 crystal lizards.
GT: LoneReaper115 Official Chaos Composite Bow of the Dark Souls message board