Place this altar in your server and see what happens when a curious unsuspecting player lights the netherrack. '/gamerule commandBlockOutput false' recommended. Make sure the bedrock is underground, so that only the altar is visible above ground.


For concerned server admins, highlight the following to see the commands contained in this schematic:

!SPOILERS!
/testforblock ~ ~4 ~1 minecraft:fire
/tellraw @a {"text":"Herobrine has joined the game","color":"yellow"}
/playsound mob.wither.death @a ~ ~ ~ 1 0.1 0.5
/playsound ambient.weather.thunder @a ~ ~ ~ 1 0.1 0.5
/effect @a 20 10 1
/weather thunder
!SPOILERS!