hammer of kamasa

three time i killed him today, made sure he died outside the lava, nothing no drop

According to https://www.pathofexile.com/forum/view-thread/3594260 this has been fixed, but the hammer did not drop for me just now.

It seems the fix was killing the boss simply satisfies the quest objective now, and you go back to Oswald to claim the reward without any quest item in hand.