Looks like the peaceful drider died anyway.
Surprisingly the caverns continue for quite a ways down past the old demon king - home to more evil monkeys, demon flamethrowers and the cute frogs. During my exploration I manage to collect a number of pyromancy tomes for Corynx too - one from the corpse of a very familiar looking pale drider. I am very surprised to run into Knightslayer Tsorig again down here - I guess the old demon managed to posses his corpse and due to his two hander hitting like a truck, I opted to run - right into the sword wielding Anor Londo knight (possibly the same one from the swamp)!
Lucky for me the two decided to attack each other, with the Anor Londo knight defeating Tsorig. Since he was badly hurt in his duel he was easy to put down. I also found out why Tsorig had such good DPS - he was wielding the Fume Knight's greatsword. I don't even have the stats to wield that thing. A number of ladders and skeleton wheel guys later, I finally reach the bloody ballista and turn it off. Hurrah for vengeance!
That is a huge ballista.
Satisfied that this area was now clear, I warped back to the bridge leading to Irithyll city only to be attacked by a dragon dog who can turn invisible!? Oh, this again huh. The AI on this one is pretty easy to abuse though, as luring it back the way I came makes is freeze and vanish (but not recover HP). A few repeats of this leads to its demise, granting me free passage into the snowy city.
Another pretty place!