His ETB is fine as long as your other creatures have undying/when a creature dies effects.
He runs pretty well in a Blood Artist/Zombies deck where you will have blood artists on board. Ashmouth comes in, you sacrifice a Geralf's Messenger and get his undying and ETB do 2 damage trigger, or sacrifice a Gravecrawler and summon him back later. In both cases, you get X number of blood artists triggers giving you a 2*X lifepoint swing.
Still, he's not very playable in standard, but he can serve a purpose as a sac-outlet.
As for the rest of that deck, I'm kind of wondering what those random humans are doing in there? Somberwald Vigilante is a pretty subpar creature given how many good 1 drops black and red have. He doesn't seem to do anything besides chump and become fodder for Ashmouth. Hanweir Lancer is nice for the first strike, but your demons already have evasion anyway.
Soulcage Fiend makes no sense in your deck because you have no source of lifegain. He is borderline good if you have a way to gain life because he is essentially creating a 0 point life differential. You turn this into an advantage for you by having sources of life gain to make up for losing 3. Other than that, he can be useful in burn decks where you just need him for the absolute 3 damage for the kill or something.
Blind Zealot is kind of hard to get use of because he has no evasion. He is better replaced by a straight up removal card like geth's verdict. Same effect except it happens instantly and you don't have to hope you manage to hit your opponent to trigger it.
All the other demons are... okay I'll just stop... Demons aren't really a playable trible to be honest. One or 2 demons in a ramp or control type deck would work, but not if your whole deck revovles around demons.