The Chosen Undead (Solaire) is a high risk, high reward character, and you'll see that mentioned frequently. He has medium combo potential, but he's played best when you're using calculated, patient attacks. He has lots of end lag on lots of attacks, but most of them have some crazy mechanics if you land them. Read your opponents' moves and punish effectively!
I tried making him sound like he came right out of the Dark Souls game, so patient play is rewarded. One of the most fun things to do in Dark Souls is play with lots of different weapon builds, so I tried to include a lot of weapon types within the character. He primarily uses a straight sword, but greatswords, spears, crossbows, axes, miracles, pyromancies, and sorcery also come into play. He has a pretty awful recovery, so getting him offstage would be your best bet. Another strategy is to bait out his very laggy moves, and punish accordingly. I put thought into nearly every aspect of this character, including sleeping animations, victory poses, etc.
Grounded Attacks
Jab: 3 sword swings.
Forward Tilt: Shield Break Kick, can cause player to trip.
Down Tilt: Kneeling, Jutting out stab attack.
Up Tilt: Sword Swing in an arc motion.
Dash Attack: Uses a spear, very long range.
Aerial Attacks
N-Air: Two slashes, one left, one right.
F-Air: Shield Bash (akin to Palutena's B-air), high knockback.
B-Air: Crossbow Shot, similar to Villager's F-air and B-air, but can't be spammed as fast.
Up-Air: Straight stab motion upwards, low knockback.
Down-Air: Plunge Attack, extremely high knockback, works similar to how it does in the Dark Souls games, high landing lag, spikes.
Smash Attacks
Forward Smash: Greatsword Swing, slow, but high knockback.
Up Smash: Black Knight Greataxe Two-Hand R2, does chip damage at the start when dragged on the ground (near the feet), then does a huge circular motion (akin to King Dedede's).
Down Smash: Wrath of the Gods, a miracle that creates a shockwave around the character, more powerful near the base, low knockback if hit at the top.
Special Attacks
Neutral B: Pyromancy. The more you hold the button in, the better the pyromancy is. The weakest to strongest pyromancies would be Fireball > Fire Orb > Chaos Bed Vestiges. This move would act similar to Greninjas, as you can't store the projectile, but it has a slight downwards arc. If the projectile hits, it does huge knockback.
Side B: Parry/Backstab. If the enemy is facing you, your character will do a parry with their shield (counter). If the enemy attacks you during certain frames of the parry animation, you then get to do a high damaging visceral attack, but there's no knockback. The enemy then collapses, (similar to Ryu's Down-B) which can lead into some combo potential. This also reflects projectiles.
If the enemy is turned away from you, and you're shoulder-length apart, your character will do a backstab instead of a parry, which does the same effect as the visceral attack after a parry. If you're too far away though, it'll work like a normal sword stab and you won't get the collapse animation.
Both attacks need to be timed well, and if you fail, there's lots of ending lag. If used in the air, it's just a high knockback backstab, no collapse animation.
Down B: Etsus Chug. An extremely high risk, high reward healing move. Etsus is basically Sunny D that heals you in the actual game. It's not able to be used in the air. You have limited etsus, but each drink heals 50%, so it's extremely valuable. The amount of etsus you have is determined by how many stocks you have or how long you play your match. For example, if you're playing a 3 stock match or a 2:00 time match, you'll have 3 Etsus for that match. If you play with 99 stocks, you'll have 30 or so etsus chugs. Drinking etsus is very slow, like the games, and if the animation is interrupted, you lose the etsus and heal no percentage. Once your etsus is depleted for the match, your character will shrug when trying to use it.
Up B: Gargoyle Lift, similar to Duck Hunt's Recovery, a gargoyle lifts up the Chosen Undead (like how they do in Anor Londo in DS1 and Undead Settlement in DS3). The wings that flap do very minimal damage, it's not the best vertical recovery, and it doesn't last very long.
Grab, Pummel, and Throws
Grab: Dark Hand, a unique magic/fist weapon that allows you to grab your opponent.
Down Throw: Lifedrain, a move similar to Snake's down throw that inflicts damage upon the opponent, and heals around 5% for the Chosen Undead. Abusing this throw results in less and less healing.
Up Throw: Simple upward throw, sword slash combo.
Forward Throw: Simple throw forward, sword slash combo.
Back Throw: Homing Soulmass, big throw behind character, then 3-5 magic balls fly towards the player (akin to Fox, Falco, and Mewtwo's throws).
Pummel: Simple sword swing
Roll
The Chosen Undead's roll is like any other character's, but has just few extra invincibility frames. If you spam it, though, you eventually have less invincibility frames than usual due to "losing stamina".
Passive Abilities
Passive Ability: Embered Up. An ability that rewards skill and calculated play. After knocking a player into the blast zones, The Chosen Undead becomes embered, like how they do after defeating a boss in DS3. This ability boosts your defense SLIGHTLY, reducing the amount of damage you take. It's nothing incredibly overpowered, it's similar to Lucario's aura or Cloud's Limit Break buffs, just with defense boosts. Once the opponent takes a stock from you, you lose the buff until you take a stock off them again. It also creates a cool, embered look on your character's armor.
Passive Ability: Shield, works exactly like Link or Toon Link's shield.
Cosmetic Passive Ability: Going Hollow. After losing a stock, your character takes the appearance of a hollow and the voice clips sound more monstrous.
Unique Death Sound: When losing a stock, the "You Died" soundclip from the DS games plays.
Taunts
Gestures are a fun little mechanic in Dark Souls, and they'd make great taunts.
Up Taunt: Praise the Sun! (This HAS to be a gesture.)
Side Taunt: Well, what is it? (Fan favorite gesture. The legend never dies.)
Down Taunt: Point Down. (Extremely disrespectful. Should only be used in rare circumstances.)
Honestly, there's a lot of gestures from the Dark Souls games that would work. I'd like Bow to be a gesture, hopefully 4 taunts could be a thing.
Special Animations
Sleeping Animation: The Sleep gesture (DS3)
Shield Break Animation: Similar to any other character, not many gestures that relate to this one.
When knocked down: Stretch Out Gesture
When on the edge: The Chosen Undead waves his arms around in circles slightly cartoonishly.
Entering the Match: Gets summoned in by a summon sign.
Victory Screen Poses
Bonfire Rest: Chosen Undead resting at bonfire
Invader Banished: Character is doing the Jump for Joy gesture with the words "invader banished" covering the screen (but not result stats).
Bow: Character does the Bow gesture
If The Chosen Undead loses: They do the Applause gesture
Victory Theme: Not sure what they could do for this. Perhaps an uplifting remix of Firelink Shrine? Maybe the riff that plays when you enter Anor Londo?
Final Smash
Darkeater Midir: Midir (dragon) flies down and shoots the stage with his Godzilla beam and/or fire. The beam stuns the players, and then shortly explodes and does incredible knockback. This final smash could literally pertain to any boss, but I really like Midir.
Costumes/Skins
Default Costume: Elite Knight Set (Swap this costume with Solaire's armor)
Catarina Set (Siegward)
Armor of the Sun Set (Solaire, this is a must)
Faraam Set (DS2 Armor)
Firelink Set (DS3 Armor)
Havel's Set (Fan Favorite)
Xanthous Set (For magic users)
Deprived costume (No armor, best way to play)
I'd love more costumes, as there's a ton of iconic armor sets from the game, but I tried picking the 8 most recognizable ones. Hopefully someone will mod in Giant Dad.
Dark Souls Stage
Anor Londo
Anor Londo is a fan favorite area in Dark Souls 1 that features a giant castle and a large elevator. The arena takes place a fair distance away from the castle, giving you a breathtaking view. On very rare occasions, Anor Londo appears very dark and has a more blue palette (This is basically what the real Anor Londo looks like, the sun and brightness is just an illusion in the game).The stage itself is a simple courtyard with no walkoffs and a couple of symmetrical platforms raised up by non-interactable pillars. You can't go under the stage. Black Iron Tarkus and other NPCs from the Dark Souls games can be spotted. The elevator in the background also goes up and down occasionally, and enemies can be spotted patrolling the castle. Two certain boss enemies actually show up to interupt the battle...
Boss
Ornstein and Smough
Occasionally, Ornstein and Smough show up, and they'll both give you quite the beating. It's possible to beat them, and beating them once will cause them to go away for the rest of the match. If you don't beat them, they eventually leave, but come back later. Unfortunately this means the stage isn't legal, unless a stage hazard toggle is implemented.
Music
Dark Souls 1 Medley (Remix)
Asylum/Taurus Demon Theme (DS1)
Bell Gargoyle Theme (DS1)
Gwyn's Theme (DS1)
Ornstein & Smough Theme (DS1)
Sir Alonne's Theme (DS2)
DS3 Main Theme (DS3)
Slave Knight Gael's Theme (DS3)
Abyss Watcher's Theme (DS3)
Soul of Cinder's Theme (DS3)
I'm being pretty optimistic here. I'd probably cut down this list in half, but I really like Dark Souls' soundtracks.
Assist Trophy
Artorias of the Abyss
This guy's a fan favorite boss, he'll just run around and swing his sword around and do some cool magic attacks.
Any boss or NPC could work for this though.
Items
Chest: similar to the chests in The Great Cave Offensive stage, some items are available inside them when attacked. The chests only have really good weapons though, like master balls and assist trophies.
Mimic Chest: Sometimes, the chests are actually Mimics, and whoever hits them takes MASSIVE damage and knockback. Hitting the chests is a super high risk, high reward chance, so be careful. If you look closely, you can tell whether it's a chest or a Mimic (look at the chest chain), but in the heat of battle, that'll be difficult to do.
Those are all my ideas for Dark Souls related things in Smash. I mostly used Dark Souls 1 references, as that's the one that'll be on the Switch soon. I definitely went overboard and honestly, I think The Chosen Undead sounds extremely OP, but there's just so much you could do with him! He has a little bit of everything, but hopefully the difficulty of his playstyle would balance his options out. LOTS of fanservice for sure, but I had a lot of fun making it despite it just being ideas and speculation. I'd love to hear thoughts, criticisms, and more ideas, so feel free to do so!