Seirei Masters are specialized planar warriors who have called upon an entity from the inner or outer planes to train alongside them, enhancing the monk and the Seirei's abilities as if they were one. Either through a blessing from a god, training in another plane, being chosen by the spirit itself or enacting a foul ritual, monks of the Way of the Seirei Master gain the unbreakable aid of a powerful ally, forming a symbiotic relationship that sometimes changes their mind and body with the influence of that creature's plane.

Planar Spirit

Your training has granted you a mystical and permanent connection with a Seirei from the inner or outer planes. When you gain this feature at 3rd level, choose a damage type from the "Damage Types" list (PHB, 196), this choice will determine the nature and damage type associated with the Seirei that will join you for the rest of your journey.

You can use a bonus action to spend 2 ki points and magically summon the Seirei in your space. The Seirei is a Small or Tiny magical entity that can occupy the same space as you and moves with you when it is in your space. Regardless of size, it has an AC of 18, hit points equal to five times your monk level and immunity to all conditions. It is also immune to the damage type you choose when gaining this feature. If it is forced to make an ability check or a saving throw, treat all its ability scores as 10 (+0). It disappears if it is reduced to 0 hit points or after you finish a short or long rest. You can dismiss it early as a bonus action.

You determine the Seirei’s appearance, which is typically related to its damage type. On your turn, you can mentally command your Seirei to move up to 15ft in any direction (no action required). If your Seirei is ever more than 15ft away from you at the end of your turn, it teleports back to your space. When you manifest your Seirei, you gain the following benefits:

• When you make a melee attack, any of your attacks can originate from you or your Seirei. When you make an attack from the Seirei, however, all the damage dealt by the attack is determined by the Seirei's damage type.
• When you hit a creature with a monk weapon or an unarmed strike, you can deal extra damage of the Seirei's type equal to one roll of your Martial Arts die. You can use this feature only once per turn.
• If you hit a creature with one of the attacks granted by your flurry of blows, you can spend 1 additional ki point to deal this feature's extra damage a second time.
• When a creature that you can see within 5 feet of your Seirei moves at least 5 feet away from it, you can use your reaction to make an opportunity attack against that creature as if you were in the Seirei’s space.

Acid Seirei

Your chosen Seirei's damage type is Acid.

Bludgeoning Seirei

Your chosen Seirei's damage type is Bludgeoning

Cold Seirei

Your chosen Seirei's damage type is Cold

Fire Seirei

Your chosen Seirei's damage type is Fire

Force Seirei

Your chosen Seirei's damage type is Force

Lightning Seirei

Your chosen Seirei's damage type is Lightning

Necrotic Seirei

Your chosen Seirei's damage type is Necrotic

Piercing Seirei

Your chosen Seirei's damage type is Piercing

Poison Seirei

Your chosen Seirei's damage type is Poison

Psychic Seirei

Your chosen Seirei's damage type is Psychic

Radiant Seirei

Your chosen Seirei's damage type is Radiant

Slashing Seirei

Your chosen Seirei's damage type is Slashing

Thunder Seirei

Your chosen Seirei's damage type is Thunder

Planar Attunement

At 6th level, your connection to your spirit's nature has deepened. You gain resistance to your Seirei's damage type. In addition, your attacks ignore resistance to your Seirei's damage type.

Acid Attunement

You gain resistance to Acid damage and your attacks ignore resistance to Acid damage

Bludgeoning Attunement

You gain resistance to Bludgeoning damage and your attacks ignore resistance to Bludgeoning damage

Cold Attunement

You gain resistance to Cold damage and your attacks ignore resistance to Cold damage

Fire Attunement

You gain resistance to Fire damage and your attacks ignore resistance to Fire damage

Force Attunement

You gain resistance to Force damage and your attacks ignore resistance to Force damage

Lightning Attunement

You gain resistance to Lightning damage and your attacks ignore resistance to Lightning damage

Necrotic Attunement

You gain resistance to Necrotic damage and your attacks ignore resistance to Necrotic damage

Piercing Attunement

You gain resistance to Piercing damage and your attacks ignore resistance to Piercing damage

Poison Attunement

You gain resistance to Poison damage and your attacks ignore resistance to Poison damage

Psychic Attunement

You gain resistance to Psychic damage and your attacks ignore resistance to Psychic damage

Radiant Attunement

You gain resistance to Radiant damage and your attacks ignore resistance to Radiant damage

Slashing Attunement

You gain resistance to Slashing damage and your attacks ignore resistance to Slashing damage

Thunder Attunement

You gain resistance to Thunder damage and your attacks ignore resistance to Thunder damage

Planar Awareness

Also at 6th level, your Seirei can magically enhance your senses. When your Seirei is manifested, you can cast the Detect Magic, Detect Evil and Good and See Invisibility spells once.

Once you use this feature, you can't do so again until the next time you manifest your Seirei.

Planar Protector

When you or a creature you can see within 30ft of your Seirei takes damage, you can use your reaction to send the spirit to their aid, granting resistance to the creature against that instance of the damage. your spirit then disappears. 
If the damage is the same as your Seirei's type, you grant immunity to the creature against that instance of the damage instead.

Planar Empowerment

At 17th level, your connection to your planar companion has reached its peak, allowing you to bolster each other's power beyond your normal limits.

When you summon your Seirei, you can spend up to 8 ki points to empower it and yourself. For every 3 extra ki points you spend, the extra damage granted from Planar Spirit increases by one Martial arts die, as shown in the Planar Empowerment table.

Planar Empowerment

Ki points spent	Extra Damage
2	1 Martial arts die
5	2 Martial arts dice
8	3 Martial arts dice