Head to the Tomb of Raithwall. Kill both Demon Walls. They should be easy with
your equipment. Activate the side stairs in the area of the first Demon Wall by
examining the area where the Demon Wall was and then save your game outside at
the Gate Crystal. Time for another weapon.

Now take those stairs you opened up all the way to the back. Once you reach
Royal Passage, go all the way down until the two paths converge. There is a
70% chance that there will be a chest here. If there is, there will be a
22.5% chance that it holds one Demonsbane. You can fight through all of the
Tallows, or just grab the chest and run. If the chest isn't there or if you
don't get the sword, reload your game and try again. If you run out of
patience, then don't worry about it. The Demonsbane is merely temporary and
will become obsolete once we get out of Raithwall. However, the fun part is
that the chest respawns, so you can grab the Demonsbane, run back to the
save point, save, reload, and try your luck again ad infinitum/nauseum.