Make your own fog of DOOM that follows a player and poisons them when they to close.

Very simple, very easy.

1. Give yourself a named egg
/give @p minecraft:spawn_egg 1 120 {display:{Name:setfog}}

2. next create a fast restone clock

3. Add the follwoing commands to your clock

3.1.
 /execute @e[name=setfog] ~ ~ ~ /summon Zombie ~ ~1 ~ {PersistenceRequired:1,CustomName:"fog",­Attributes:[{Name:generic.followRange,Ba­se:40}],Equipment:[],Invulnerable:1,Sile­nt:1,ActiveEffects:[{Id:2,Amplifier:2,Du­ration:2000000,ShowParticles:0b},{Id:14,­Amplifier:1,Duration:20000000,ShowPartic­les:0b},{Id:18,Amplifier:25,Duration:200­00000,ShowParticles:0b}]}

3.2.
/tp @e[name=setfog] ~ -200 ~

What this does is summon a invisible fog zombie and then killing the villager you just placed.

Next is the foggy DOOM bit

4.1.
/execute @e[name=fog] ~ ~ ~ /particle cloud ~ ~1 ~ 10 1 10 0.01 1000 force

4.2.
/execute @e[name=fog] ~ ~ ~ /effect @a[r=5] minecraft:poison 1 1

and you done.

you now have a rolling fog that will damage you.

You can also grab teh one command install here: pastebin.com/JuFEX8GS