Killing Koschei the Deathless; Killing market guards; Dagannoth (lv 90); Dagannoth Prime; Lanzig in Rellekka
Notes:
If you manage to kill Koschei the Deathless, he will drop this blade, although this is a very difficult task. The market guards in Rellekka drop the blade, so don't worry about wasting potions on Koschei.