To make my lava definitely kill people I really upped the damage. It triggers over and over (most people should die on 1-3 hits), and it knocks you down so you're just crawling on all fours in lava with no hope of getting up... yay :P Doesn't look as good as I'd like (would have liked it to be more of a continuous burn that continues after death), but serves the purpose. I had it spawn a fire with smoke, but when triggering over-and-over it ended up as too much if I pushed my invincible follower into it... :P