No mere swordsman, a yokai duelist has a connection to the Spirit Realm which allows them to summon creatures called yokai to fight alongside them and serve them. 

Sacred Weapon

Starting at 3rd level, you can connect a weapon to the Spirit Realm. The Spirit Realm overlaps with the physical multiverse and is filled with marvelous and terrible beings.  When you finish a short or long rest, choose one melee weapon in your possession, including your unarmed strike.  You can use your Yokai Strike feature with that weapon.  You can only have a single sacred weapon at a time.

Yokai Strike

Starting at 3rd level, you can briefly conjure yokai to aid you in battle.  Once per round, when you hit an unwilling creature with your sacred weapon, you can perform a Yokai Strike.  Roll a d20. You perform the Yokai Strike associated with that number.  Unless otherwise specified, the target is the creature you hit.  

If the target has to make a saving throw, the DC is 8 + your proficiency bonus + your Intelligence, Wisdom, or Charisma modifier (your choice).

You can use this feature three times, and you regain any expended uses when you finish a short or long rest.

Yokai Strike Results

1: Basan's Blast. A large chicken-like creature exhales a ghostly flame over the target. It must make a Dexterity saving throw. If it fails, until the end of your next turn, the creature glows. While glowing, it cannot benefit from being invisible, emits bright light in a 5-foot radius and dim light for an additional 5 feet, and location-obscuring spells like Blur and Mirror Image have no effect on it.

2: Boroboroton's Embrace. An animated futon mat tries to crush the target. It takes an extra 1d6 + half your fighter level bludgeoning damage.

3: Daidarabotchi's Reposition. A colossal giant picks up one creature of your choice you can see within 30 feet of you and moves it to an unoccupied space you can see within 10 feet of the target. This movement doesn't provoke opportunity attacks. If unwilling, the target can make a Strength saving throw, resisting the effect on a success.

4: Hakutaku's Salvation. A white yak-like creature with a humanoid face moves to a creature you can see within 30 feet of you and uses its medical lore to aid it. The target regains 1d6 + half your fighter level hit points. Any excess healing becomes temporary hit points, which disappear after 1 minute.

5: Jorōgumo's Feast. A spider-woman moves to the corpse of a creature you can see within 30 feet of you and lays her eggs inside it. Constructs, undead, and creatures without an organic body (DM's discretion) are unsuitable hosts. If there is no such body, the jorōgumo instead angrily bites the target, dealing an extra 1d6 + half your fighter level poison damage.

At the start of your next turn, if the jorōgumo laid eggs, her children emerge from the body. The children have the statistics of a swarm of insects (spiders), except that its attacks count as magical for the purpose of overcoming resistance and immunity to nonmagical attacks and damage, and it adds your proficiency bonus to its attack and damage rolls. The swarm acts during that turn and then dissipates.

6: Kamatachi's Whirl. A weasel-like creature with sickles for limbs appears in a whirlwind and slices the target. The target takes an extra 1d6 + half your fighter level slashing damage.

7: Kappa's Trick. A mischievous water spirit that resembles a turtle with a bowl-shaped head, a kappa attempts to distract the target. It must make an Intelligence saving throw. If it fails, the next time a creature attacks the target before the end of your next turn and misses, it rolls a d4 and adds the number rolled to the result, potentially turning the miss into a hit.

8: Kirin's Charge. A juvenile kirin, a noble spirit resembling a golden unicorn, rushes the target and erupts in a flash of light. It takes an extra 1d6 + half your fighter level radiant damage.

9: Kitsune's Confusion. A fox-like humanoid strikes at the target's mind. It must make a Wisdom saving throw. If it fails, once in the next minute, if it succeeds on an Intelligence, Wisdom, or Charisma check or saving throw, you can roll a d4 and subtract the number rolled from the result, potentially turning the success into a failure.

10: Koi King's Wrath. A magical koi appears and flops on the ground in an unoccupied space you can see within 30 feet of you. It cannot be harmed in any way. You can use an action while within 5 feet of the koi to kick or otherwise physically abuse it. If this is done, the koi transforms into a massive sea serpent that unleashes a powerful beam of energy at any creature or object that you can see within 30 feet of the serpent. The target takes 1d6 force damage for every two fighter levels you have, rounded up.  The koi disappears after this attack or after 1 minute.

11: Komainu's Protection. A pair of lion-dogs appear in an unoccupied space you can see within 30 feet of you. Until the start of your next turn, all creatures of your choice receive a +1 bonus to AC as long as they are within 15 feet of the komainu.

12: Onryō's Revenge. A vengeful spirit tries to snuff out the target's life. It takes an extra 1d6 + half your fighter level necrotic damage.

13: Raijū's Descent. A lightning beast pounces on the target from above like a bolt from the blue. It takes extra lightning damage equal to 1d6 + half your fighter level.

14: Samebito's Snap. A shark-man lunges at the target with gaping jaws. It takes an extra 1d6 + half your fighter level piercing damage.

15: Shinen-gaki's Slam. A burning spirit with a dark spot for an eye tackles the target. It takes an extra 1d6 + half your fighter level fire damage.

16: Tenko's Blessing. A divine fox grants a boon to one creature you can see within 30 feet of you. Once in the next minute, when it fails an ability check or saving throw, it can roll a d4 and add the number rolled to the result, potentially turning the failure into a success.

17: Tōfu-kozō's Meal. A boy carrying a plate of tofu appears in an unoccupied space you can see within 30 feet of you, bemused by the combat. He cannot be harmed in any way. You and any creatures of your choice can use an action while within 5 feet of the boy to eat the tofu. The eater gains a number of temporary hit points equal to twice your fighter level for 1 minute. The boy disappears after the tofu is eaten or after 1 minute.

18: Ungaikyō's Reflection. A cursed mirror appears, reflecting a warped and monstrous version of the target. It takes an extra 1d6 + half your fighter level psychic damage.

19: Wanyūdō's Prize. A burning oxcart wheel bearing the tormented face of a man tries to steal the target's soul. It must make a Constitution saving throw. If it fails, once in the next minute, if it succeeds on a Strength, Constitution, or Dexterity check or saving throw, you can roll a d4 and subtract the number rolled from the result, potentially turning the success into a failure.

20: Yuiki-onna's Breath. A spirit of a snow woman blasts the target with an icy wind. It takes an extra 1d6 + half your fighter level cold damage.

Yokai Expert

At 3rd level, because of your deep familiarity with spirits, you have advantage on Intelligence checks to recall information about celestials, elementals, fey, fiends, and undead, on Charisma checks made against them,  and on Wisdom (Insight) checks made against them.

Conjure Minor Yokai

Beginning at 7th level, as an action, you can summon a minor yokai in an unoccupied space you can see within 30 feet of you. Minor yokai are Tiny creatures that cannot take the Attack action, use the Help action to grant advantage on an attack roll, or harm creatures. They obey your commands otherwise.

Minor yokai will rarely play a role in combat. If the situation arises, they act on your turn, including the turn summoned, and unless otherwise specified, have a movement speed of 15, an AC of 10, and 1 hit point. All of their ability scores are 10, save their Strength, which is 4. Minor yokai cannot be affected by size or shape-changing spells. Undead yokai are immune to poison.

Minor yokai remain for 1 hour or until you dismiss them (no action required). 

You can use this feature three times, and you regain any expended uses when you finish a short or long rest.

You can conjure a number of different types of minor yokai equal to your Intelligence,  Wisdom, or Charisma modifier (your choice). 

Minor Yokai Options

Amikiri. A mixture between a serpent, bird, and lobster, an amikiri can use its action to cut through nonmagical nets, ropes, and similar material within 5 feet of it, freeing a creature restrained by such material. The amikiri disappears after doing this. An amikiri cannot free creatures restrained through magically animated plants or magically created webs. It is an elemental.

Biwa-bokuboku. A humanoid with a head in the shape of a biwa, a stringed-instrument, it can play itself. It can take the Help action to give advantage on a music-related Charisma Performance check and has a +8 bonus to Charisma (Performance) checks. It is a fey.

Chōchin-obake. A paper lantern possessed by a ghost, it sheds bright light in a 10-foot radius and dim light for an additional 10 feet. The light can be lowered (no action required). It has a flying speed of 30 feet. It is undead.

Enenra. A cloud of smoke and darkness with a vaguely humanoid face, an enenra lightly obscures the space it occupies. It can move into another creature's space, and can pass through any space smoke could. It has a flying speed of 30 feet. However, they dislike each other and won't move into the same space. It is an elemental.

Jikininki. A dark, shapeless spirit, a jikininki can spend 1 minute to devour the corpse and possessions (but not any magic items) of a creature size Medium or smaller. It disappears after doing so. It has a flying speed of 10 feet. It is a fiend.

Noppera-bō. This spirit appears as a translucent, faceless humanoid. You can use your action to direct it to interact with objects like the hand created with the Mage Hand cantrip, save that it can carry up to 15 pounds. It has a flying speed of 15 feet. It is undead.

Kodama. A kodama is a tree-shaped spirit. You and any plant creatures within 30 feet of the kodama can communicate as if you shared a common tongue. It is a celestial.

Tanuki. An affable and loquacious racoon-dog spirit, a tanuki appears with a bottle of sake. One creature that shares the sake with the tanuki gains a bit of good luck. The next time within 1 hour that the drinker would roll with disadvantage, they can ignore the disadvantage. The tanuki disappears after sharing its sake. It is a fey. Beside the yamabiko, tanuki are the only minor yokai able to speak.

Yamabiko. An ever-grinning cross between a dog and monkey, a yamabiko can expertly imitate sounds that it hears, including voices. A creature that hears the sounds it makes can tell they are imitations with a successful Wisdom (Insight) check opposed by its Charisma (Deception) check. The yamabiko has a +8 bonus to Charisma (Deception) checks. It is a fey.

Yatagarasu. An otherwordly three-legged crow, a yatagarasu can use an action to cast the Guidance cantrip with a range of 30 feet. The yatagarasu disappears after casting Guidance, but the cantrip remains in effect. It has a flying speed of 50 feet. It is a fey.

Improved Sacred Weapon

Also at 7th level, your sacred weapon counts as magical for the purpose of overcoming resistance and immunity to nonmagical attacks and damage.

Double Strike

Starting at 10th level, you have some control over the type of yokai you conjure in battle. When you use your Yokai Strike feature, you can roll two d20s and choose which Yokai Strike to perform.

In addition, you can choose to perform both Yokai Strikes. Once you use this feature, you must finish a short or long rest before you can use it again.

Improved Double Strike

At 15th level, you can activate your Double Strike feature to perform both Yokai Strikes twice between rests.

If you roll any number except 1 or 20 twice,  instead of performing the normal Yokai Strike twice, you can choose to instead summon a tengu, a trickster spirit. It teleports a willing creature you can see within 30 feet of you up to 30 feet away into an unoccupied space of your choice.

If you roll two 20s, instead of performing the normal Yokai Strike twice, you can choose to instead summon an adult kirin, a powerful holy spirit. If you do, choose one creature you can see within 30 feet of you. It has advantage on attack rolls, ability checks, and saving throws until the start of your next turn.

If you roll two 1s, instead of performing the normal Yokai Strike twice, you can choose to instead summon a greater oni, a dangerous demon that unleashes a woeful curse. If you do, choose one creature you can see within 30 feet of you. It must succeed on a Charisma saving throw, or have disadvantage on attack rolls, ability checks, and saving throws until the start of your next turn.

In addition, when you roll initiative and have no uses of your Yokai Strike feature left, you regain one use of that feature.

Master Duelist

At 18th level, when you would roll a d20 to randomly determine which Yokai Strike ability is activated, you can instead choose which ability activates. 

You can use this feature four times, and you regain any expended uses when you finish a long rest.