Summon the Black Smoke Monster mob

Summon near you:

/summon Bat ~10 ~10 ~10 {CustomName:"BSM",Invulnerable:1,Silent:1,ActiveEffects:[{Id:1,Amplifier:1,Duration:2000000,ShowParticles:0b},{Id:14,Amplifier:1,Duration:2000000,ShowParticles:0b}]}

Summon randomly in the world:

/execute @r[type=!Player,c=1] ~ ~ ~ /summon Bat ~ ~1 ~ {CustomName:"BSM",Invulnerable:1,Silent:1,ActiveEffects:[{Id:1,Amplifier:1,Duration:2000000,ShowParticles:0b},{Id:14,Amplifier:1,Duration:2000000,ShowParticles:0b}]}