There are HoS ending spoilers below.
The Viper Venomous Silver Sword is a unique level 39 sword with very powerful attributes. You can only find this sword if you choose to intervene and save Olgierd. When you intervene then you will be transported to another world and have a limited amount of time to find the answer to a riddle. In this world you can find this sword. Watch the video below to find out where to go.
If you cannot watch the video then run left after you cross the bridge. Keep heading left and you will eventually see a sword sticking out of a stone. It will be guarded by the spectre of a Leshen that you must defeat before you can interact and take the sword.
I hope the sword serves you well! If you wish to support me then please consider downloading and trying the recommended game from the sidebar. I change the game every month and only recommend games that I enjoy playing.