Bel

Balor's mostly there for extra damage and being a decently costed 5/5 demon. I get wanting to cut Mutalith Vortex Beast since it honestly doesn't do much, but why Tzaangor Shaman, Dark Ritual, and Scroll Rack?

Also, what would your advice be if I want to focus more on the spellslinging aspect of the deck? 

Right, one last thing I forgot to mention: something important for all my decks to have other than value generators is resilience- the ability to quickly come back from boardwipes and to protect itself against interaction. Yawgmoth can sac for value and has plenty of reanimation spells to use, Ephara can blink stuff out, and Breena has some reanimation, indestructible enablers, and benefits off having stuff die. Abaddon currently doesn't seem to have any way to protect itself or quickly rebuild.

I did just realize, though, that the Doomsday+Oracle+Consultation combo isn't completely reliable in the deck since I might cascade into Oracle or Consultation by accident, so it probably isn't the best option. Have any other ideas for a good game-ender? Maybe an infinite turn combo or something? Never played a cascade deck before, so my main idea with it is to just get big value off it.

Regarding what to cut, I'm currently looking at The Horus Heresy (sad, I'd honestly rather keep it), Divide by Zero, Unsubstantiate, Delayed Blast Fireball, and Demonlord Belzenlok. If I cut the Dommsday stuff, that'd leave two cards left to cut, and I've been looking at Helbrute and Lucius the Eternal. Like with the Heresy, though, I'd rather keep them because they're pretty cool, and their haste and ability to recur themselves if removed seems like it could come in handy in a pinch.

I plan to play the base precon itself for a bit to decide what I like best, but right now I'm mostly leaning towards focusing on the demon theme. It's the easiest to stay on flavor with, and I'm more fond of creature-heavy strategies than ones that rely on mostly casting noncreature spells. The idea I had going in is to deal damage with creatures to be more proactive AND to have mana to cast my other spells, rather than using that mana to cast burn spells to meet cascade requirements. That idea is also why I have passive damage effects like Sanctum of Stone Fangs and Stormfist Crusader to rack up some damage without doing anything.

As far as the Doomsday package goes, yeah, it's not a dedicated Doomsday deck and more or less just shoehorned in there to act as a killswitch for when I really need to instantly close out a game. Most of the time, I prefer to play things out and have longer games without threatening a win at every moment, but if I see someone pulling too far ahead or am falling too far behind myself, it's nice to have an instawin combo to fall back on.

It's ok lol, I was able to learn the game and built a few decks already. Thanks, though!

Very nice! I've never found a shiny in the wild before; the only time I chanced upon one in-game was when I was breeding Honedges for perfect IVs in Pokemon Y. Sadly, its IVs were pretty mediocre, but it was still kind of cool when it happened.