Maybe for soy milk, you should be able to hold the command down and spin repeatedly without stopping to compensate for lower damage.

spirit sword + mom's knife:
attaches a knife to the end of the sword forgotten + mom's knife style, when doing a spin attack, you throw the knife out which causes the damage of it to go up to 6x

also maybe more mom's knife synergies next?
that would be fun!

Sulfur: The synergies you added with brimstone

Pupula Duplex: The projectile is wider and pierces

Monstro's Lung: Charge attack creates large amount of sword projectiles

Kidney Stone: Release a lot of sword slashes

Neptunu: More sword swings I guess (I feel like somebody else can make a better idea than this)

Charm of the Vampire - Sword Swings can fire while at 1 or less Hearts, and killing champions give health
Steven - Sword Swings leave a delayed black ring before triggering.
Pinking Shears - The sword becomes pinking shears. Fires twin bouncing beams at full health
Crack The Sky - While Crack The Sky is not fully charged, killing champions give 1 point of charge. When using Crack The Sky, your blade fires CTS shots for 7 seconds
Cricket's Body - There is a chance that Spirit Sword will break. Upon breaking, shards will fly all over the place. Restores the blade every 5 seconds
Dead Eye - Sword Beams home in on enemies, but you can only strike enemies with Sword Beams
Glowing Hourglass - Upon triggering, and returning to the same room, afterimages of your sword slices trigger whenever an enemy walks over them
-> Auri Glowing Hourglass - Shares the same effect as Glitch Knife
My Shadow - Your shadow will chase down and impale enemies with the spirit sword.

Modded:
Not a Bullet (Revelations) - Sword Beams can be fired regardless of Health
Cursed Grail (Revelations) - Slaying Champions fills the Cursed Grail
Broken Oar (Revelations) - The sword is replaced by the Oar. Enemies can only be struck by the tip of the Oar, and fires tears upon landing a successful hit
Aegis (Revelations) - Sword Beams fired have a chance to become shield beams, deflecting any shot fired at them
Birth Control (Revelations) - The sword becomes a hanger. Gains more power from familiars despawned by Birth Control
Virgil (Revelations) - Bury The Light
Mended Knife (Cursed Collection) - The sword can now access the Corpse
Nugget (Golden Collection) - The sword becomes a Pickaxe, and beams fired behave in a Monstro Shot
Mother's Spine (Rotten Collection) - Sword swings have a chance to trigger Reverse High Priestess towards all enemies. Mom's Foot/Mother's Knife cannot harm you
Haunted Rose (Sewage Collection) - Create a Rainmaker familiar who wields a secondary sword with you
Jar of Air (Toybox Collection) - The sword becomes the Zenith, and fires rotating blades equal to the amount of health you have
Old Knife (Epiphany) - Upon firing a sword beam, you teleport to the location it stops at, piercing through enemies similar to Decent
Glitch Knife (Auri) - Gain more power scaling with HP. Picking up items may instead give bonus Red Hearts.
Little Auri/Princess Baby (Auri) - Gives the familiars blades of their own.
The Memory (Auri) - Spirit Sword instead becomes Glitch Knife
Sigil of Aamon (Demonic Sigils) - Every once in a while, you will strike all enemies with your sword
Sigil of Belial (Demonic Sigils) - Sword Beams consume half a heart to fire. You can fire 6 times before the beams take away your health.
Dionysius (Tainted Treasures) - Upon striking an enemy with the sword hanging above them, if the hanging sword kills them, the sword reappears above another enemy
Dryad's Blessing (Tainted Treasures) - The sword becomes the Terra Blade. Increase radius of sword beams
Lil' Slugger - (Tainted Treasures) - WIELD A MOTHER ♥♥♥♥♥♥♥ CHAINSAW THAT FIRES SAWBLADES

Decap Attack!:If head gets recalled by the item,it does sword spins until it reaches back to the body!
Buzzing Magnets(Tainted Treasures):Repulses enemies while charging the sword,similar to enemies being effected by the "Replusion" debuff
The Bottle(Tainted Treasures):If the bottle breaks,the sword will strike from above and shoot sword beams in 8 directions.
Glad Bombs(Tainted Treasures):Bombs do sword spins until they explode.
Cross of Chaos(Repentance Plus):Emits a harmless beam that has a chance to inflict enemies with the "Cripple" debuff

Strange attractor should repel the projectiles around you slightly every 1-3 seconds of charging, just like the enemies do!

Did as many base-game passive item synergies I could think of

https://docs.google.com/document/d/129cDJ3kdZRWFXAERoQCXdpuKnexDxdXwFe7uuNH6BQs/edit

what if the forgotten got both a sword and a club and it at like you had 2 swords and when you charge you throw the club and spin and the same with t Samson

Oh my god I'm so surprised this isn't already one, but death's touch should totally make the sword into a larger hitbox sythe like how mom's knife does it with forgotten's bone club.

I think for Brimstone would be better if the laser fires after a charged slash. Like you spin and then fire a brimstone laser. Would be pretty cool

Could you make a download link for this please I would really appreciate it 🙏

Whilst holding the spirit sword The Forgotten will shoot sword projectiles whilst swinging his bone club, if his hp is full. Also he performs a spin attack along with the bone toss upon unleashing a charged hit.

This change wouldn't fundamentally change the Forgotten, but he already is a melee character, so giving the guy the extra functionality of the sword makes sense.

Oh, also those changes should also apply to the tainted version as well.

if you see this then: death's touch - the sword is looking like scythe, when you attack enemy it inflicts fear, when you swinging with full hp its summons scythe, when you do spin attack it would summon bigger scythe

hopefully I can put in a suggestion: Forgottens bone: there surprisingly isnt a synergy for this but for some reason mom's knife synergrizes with it??? anyway replaces forgotten's bone with a much (bare with me) a longer bone and now acts as spirit sword!