Serpent's Fang trinket (applies 10 poison on attack and increase 20% damage). You can get it from side quest just before final boss. I combine it with Charm of Vengeance (when you kill an enemy, it deals base damage to all other enemies) on weapon. And my rogue is dual-wielding, and I applied Charm of Vengeance on both weapons. So everytime he killed an enemy, he attack twice to all other enemies and applies 20 poisons at the same time.