Dig up a Corpse Worm in the Ruins of Lordaeron courtyard and use it as bait to catch a Corpse-Fed Pike. Corpse-Fed Pike can be fished up in Brightwater Lake, northeast of Undercity.
You've fished in Brightwater Lake, yes?
The prize catch there is one of the lake's massive corpse-fed pikes. However, they only bite on a very specific type of bait... corpse worms.
Search the graves in the courtyard up top and you should find some.