An El rune from the Countess.
The thing you are looking for, but not the right stuff.
i.e. ordinary Feral Claws with the wrong skills.
Been looking for not just one, but TWO(well either one and a suwayya) Feral claws with +3 venom...
... one month later no such luck. tried hell cows to pit runs to countless chaos runs... nothing.
Ask the assassin forum if you can shop for the specific mods you're looking for in act 5.