A poison build in Dark Souls 1? You're asking for a real challenge there, champ.
This, however, appears to be a run-of-the-mill quality build with toxic mist where the dark flame ought to be. Let's fix that.
The Gravelord Greatsword is a curved greatsword that inflicts toxic buildup with each hit. The weapon's damage itself isn't that great, but the moveset is excellent and you're using it for the secondary effect anyways. The Dark Silver Tracer (DLC) is a dagger with an extremely high critical damage modifier and a little-known secondary toxic effect on its R2 thrust. The buildup isn't a whole lot, but used effectively you could put some pressure on someone they weren't expecting while you're circling around them, just daring them to use an estus or moss clump so you can finish them off.
But if you really want to use a poison build, you really, really ought to be using the Guardian Tail weapon. Yes, I know, whips are terrible weapons. Except in this case, you aren't worried about the damage output and are worried more about the poison buildup. Take the Guardian's Tail, slap some Rotten Pine Resin on there, and it'll poison almost anyone in a single hit. This is best used with the two-handed R1 thanks to its absolutely enormous hitbox. The Large Club is another weapon that goes very will with Rotten Pine Resin, and its one-handed R2 has another gigantic hitbox with an added lunge to give it longer than normal reach. Not to mention the Large Club is also a serviceable strength weapon, so any hits from it will hurt.
So, there're your weapon options. You can go 40 strength and 10 dexterity with Large Club and Guardian's tail for a very effective one-shot poison setup, or you can pick up the Gravelord Greatsword and rely on its solid moveset to slowly grind up toxic, that sort of thing. In either case, your opponents will definitely try to back up to use a moss clump or estus so you definitely need to be on their *** and ratchet up the pressure to either keep them from doing it and watch them slowly die, or backstab them when they do for a quick win.
In regards to backstabs, the Large Club is probably the way to go for that one. Combined with the Hornet's Ring, its backstab damage is through the roof and each hit in the animation will inflict poison damage. If they survive the backstab itself, they sure won't survive the poison damage afterwards.
I'd actually recommend dropping either strength or dexterity and raising vitality to 90 or so. If your build is supposed to be about inflicting your opponent with poison and watching them slowly die, then you're going to need some extra Hp while the "slowly die" part works its magic. Maybe even a touch of faith to use Hp-regen miracles. All in all, this could be a very fun playstyle.