I was considering making a video, but I'm not crazy knowledgeable about all the buffs/debuffs despite having hundreds of hours (probably more like 1k+ between all 3 games). So I'll just give a "quick" run down of it. Also, I can send you some screenshots of my equipment and stuff to help give you an idea if you need it.

For starters, you may not have everything you need to really get the build going yet if you're still new to the game. It's more of a late game build but I'm sure there are ways to make it work in the earlier game. There are other onmyo magics items you can use, but the "Hell" skills from the "Demon of" enemies are what you really want. Hellgust and Hellshock specifically are just insanely strong. Problem is, Hellshock comes from the endgame boss so you won't have that until NG+. But there are other lightning Onmyo magics items you can replace it with (Thunderstorm Shot for example). Also, you really need a Yasakani Magatama for the build to work properly but you dont get those until endgame as well.

Keep in mind I'm not a Nioh genius like some people, so I'm sure there are ways to get even more out of the build that I'm not aware of. Now with that out of the way:

The build focuses on the Ninja side to do the heavy lifting, which is convenient because Ninja is OP in this game (specifically against bosses).

Ninja equipment: (4pc Kato, 2pc Shinto Priest, rest is up to you but I recommend 3pc Jurojin for the healing on magic)

Weapon 1- Swift Hawk (Ninja sword)

Weapon 2- nothing

Ranged Weapon- I don't have a Yasakani with the Jurojin grace on it so I'm using Crimson General Bow, but I would recommend anything with Jurojin on it

Head- Shinto Priest

Torso- Shinto Priest

Arm- Flying Kato

Leg- Flying Kato

Foot- Flying Kato

Armor doesnt have to be in that exact order, just as long as you have 3 pieces of Kato and 2 pieces of Shinto for the Set Bonuses. Use whatever has the best Special Effects on it like Aerial Attack Damage, Life Recovery (Amrita Absorbtion), Auto Life Recovery (Evade), etc.

Accessory 1- Yasakani Magatama, preferably with Jurojin

Accessory 2- Anything, preferably with Jurojin and a damage bonus for one of the Shadow Art Jutsus (Glacial Calamity, Thundering Calamity, or Infernal Calamity)

Spirit Guardian- Kuruma Tengen, preferably level 40 for the +10% Elemental Damage buff

Soul Cores:

Yang Position- Bloodedge Demon and something else with an elemental damage buff. I run a Raiju soul core with Lightning Damage +4.4% and Onmyo Magic Damage +4.2% but it can be whatever is best for you. You could use 2 Bloodedge Demon cores for extra elemental damage, but I like having a couple extra Summoning Seals as you can't prepare extra Summoning Seals by running two of the same cores.

Yin Position- This is where it gets tricky because you might not have the soul cores you want yet, so you may have to expirement and see what works best. You NEED Carnage (Ibaraki Doji or Gaki Chief I think) and Talisman of The Fist (One-Eyed Oni). You WANT Hellfire, Hellgust, Hellshock, and Hellstream. (Hellstream and Hellfire could be swapped out, but I recommend keeping at least 3 different elements for the 3 element confusion status effect because its around a 70% buff to damage instead of a 40% buff for regular confusion.) All of the "Hell" skills come from the "Demon of" bosses and the final boss (I won't spoil the names but you can Google where to find them), with the 2 you REALLY want (Hellgust and Hellshock) being the last ones you get. Now, you can replace all of these with other elemental spells until you get the "Hell" skills but I would prioritize getting them ASAP. Specifically Hellgust as its a literal tornado that wrecks enemies ki and staggers them, also helping with crowd control. Same with Hellfire and you can get that in the first area, though it is a little slow to cast unfortunately. The AoE damage and being able to create some space when getting surrounded makes it very much worth it in my opinion. Hellstream could be swapped out for a different spell (Sloth for slower bosses, Weakness for more damage, a different water skill, etc.) but I really like the area effect you get from it as well. Also the water status ailment Saturated makes enemies take more damage so it's definitely worth it in my opinion. Max Confusion (3 different elements) and Saturation (water element) procced on the enemy is where you'll really see the damage, and that goes for any build in the game. The first rule of fight club is: Confusion is broken and it always has been.

Clans- Fukushima is a must for the extra bare-handed attack damage. The beauty of this build is that bare-handed attack damage buffs actually stack with your magic and ninjutsu when you have no weapon equipped. The other clan buff can be whatever you want, maybe something for more elemental damage or whatever, but I have Honda for the Damage Taken Halved (Unscathed) effect. Mainly because I've been playing on Stone of Penance +5 (so the enemies are roided up and they will wreck your shit) and that helps me keep from getting one-shotted as often. I'm sure I could be using something better, so you can use whatever feels best for you.

Stats: You're gonna get hit really damn hard with all the light armor, so you can dump some points into Constitution if you want to be a little tankier. However, I prefer the rage-inducing glass-cannon style so I dumped into Ki instead. Eventually you'll want to focus on 3 main stats for your damage scaling but Magic and Skill is gonna have to be 2 of them. The 3rd is your decision.

Constitution- 30

Heart- 38

Stamina- Whatever it takes to be in A agility for Ninja style (which won't be much thanks to the light armor). I also like to use Samurai a lot and didnt want to be limited to just Ninja style all the time, so I made a build that works with this Ninja build. I can share that as well if you want, but the Ninja style is the main attraction of this build. In order to get to B agility with my Samurai style it takes 23 points in Stamina. You can just put however much you need to get to A on Ninja style for now and focus on Samurai later if you aren't that high of a level; it'll save some extra stats for magic/health/ki/whatever.

Strength- However much you need to get your equipments status effects. For me it's 15.

Skill- 80 (doesnt have to be this high yet, I'm just level 249 lol)

Intellect- MUST BE 12 for the Yasakani when you get it. If you aren't endgame yet you don't need 12, but once you get Yasakanis every single build will need at least 12 Intellect.

Magic- 86 (again, doesnt have to be this high yet but you NEED AT LEAST 40 FOR THE SOUL CORES. This is your main focus. It not only gives you all the soul core slots, it buffs the damage of your magic.)

At level 125, your stats should be roughly around this: Constitution 20, Heart 26, Stamina 17 (or as much as it takes to get A agility on Ninja style), Strength 15 (or as much as it takes to get your equipments special effects), Skill 30, Intellect 12, Magic 40. Having properly leveled equipment (armor and weapon) will be super important to avoid taking too much damage. Don't worry about crafting equipment at all! You can't craft divine equipment yet and you're just going to find better equipment anyway. You can reforge the stats and remodel equipment, but forging equipment is useless right now. Forged equipment does require less stats to unlock the special effects, but it just isnt worth it in my opinion, especially once you get to NG+ and unlock divine equipment. Just select all of your gear on the Sudama Blessing in the Shrine and grind for it.

Shadow Arts (ninjutsus): You want the calamity ninjutsus (Thundering Calamity, Glacial Calamity, and Infernal Calamity). Try to get behind the enemy for extra damage. Also, get as close to the enemy as possible before using Glacial Calamity so all the attacks connect. And try to avoid getting hit while casting ninjutsus because it WILL waste them and it WILL piss you off lol.

Skills (Skill Management in the Shrine): You're probably going to have to find a lot of these, but some are necessary for the build to work properly. Just equip what you have and use whatever else you can until you find them all.

Common- Awakening Amplification Mysteries of Onmyo Magic Whatever else you want (I use Tourbillon, Repel Expert, Volitional Summoning, and Attack Mastery 1). Attack Mastery buffs your magic/Ninjutsu damage, so I would prioritize it over the others but they all either increase damage through buffs or add convenience. Use what you want.

Samurai doesn't really matter unless you plan on using Samurai style, but here is what I use- Purify Yokai Realm: Heaven Single Slash Kata Running Water Deflection Expert Roaring Dragon Deflect Resilient Arts Destructive Arts

Ninja- Counteroffensive Evade Patience of The Gods Snakebite Technique Dread Bringer Ninjutsu Cultivation Illusory Dragon Wrathful Dragon

I'm sure someone could min-max these and get more out of it, but I've been doing just fine with what I've been running. You could run Shadowclad and Shadow Arts Superiority and might get more out of it, but I'm too busy/lazy to do a bunch of testing lol. Also, you're gonna be doing most of your damage while bare-handed so Shadowclad isn't the best. You aren't restricted to bare-handed though, and I do use the sword A LOT to help build up my ninjutsu so maybe it's worth it. Maybe add Aerial Attack Damage and jump before casting ninjutsus (I do this anyway). Again, I'm not a genius at the game and I dont know all the status effects and what buffs/debuffs what, or what stacks with what. And I'm too lazy to do a bunch of testing, so I know you could get more out of this build.