Weapon Saints have specialized in fighting with a specific weapon type. Day and night they train to become absolute masters with this one weapon. Doing that they becoming absolute killing machines as long as that weapon is in their hands and they have an almost friendship like relationship to it. Of course that has one big disadvantage: If they get separated from said weapon they lose their one power.

When it comes to the weapon itself there is no limitation. Be it a sword, an ax, a throwing knife or a bow. They can specialize on any weapon type and train to become a master in its use.

Specialization

Choose one weapon type, no matter if simple or war weapon. Whenever you attack with this kind of weapon you can add half of our fighter level (rounded up) to your attack throws. Additionally, you can only use weapons of this type for the other features of this subclass.

Bless Weapon

By choosing this archetype on level 3 you learn how to bless your weapon. It takes 1 hour to perform this blessing on the weapon of your specialization. With it your weapon gains the following effects:

• Attacks with this weapon count as magical to overcome resistances. 
• You have expertise with that weapon.
• You can use a bonus action to call your weapon into your hand. You can do so as long as the weapon is within the same plane of existence. 

This effect is only active on this one weapon and only when you hold it. For everyone else it is just a normal weapon. The effect holds until the weapon is destroyed or you bless a different weapon.

Magical Enforcement

With level 7 your constant training increases the magical power of your weapon. Your blessed weapon has a permanent damage bonus of +1 of its damage type. On level 10 this damage increases to +2 and on level 13 to +3. 

Enchanting Enchantments

Beginning with level 10 you can adapt your weapon to your personal taste. You have to use up 1d12 hours to do so. These adjustments can only be of an aesthetic nature (for example engraved runes or beautiful engraved patterns). With these ornaments you can add one of these following effects to your weapon:

Elements. The damage type of your weapon is changed into one of your choice: Fire, cold, lightning, thunder or acid.

Revenge. You can use your reaction to perform a counter-attack whenever an enemy attacks you.

Speed. Your base walking speed increases by +10 feet.

Defense. As long as you are holding this weapon in your hand your AC increases by +2.

You can change the art of enchantment by using up another 1d12 hours to retune it. These effects are only active when you hold the weapon.

High Weapon Blessing

On level 15 you are now able to perform a high blessing for your weapon. After performing your normal blessing you can use up another hour and 100gp to increase the power of your blessing. Additionally to the effects of the normal blessing you get the following:

• You can choose a second enchantment effect from the "Enchanting Enchantments" feature. You do not have to use any additional time to do so, the effect is immediately active.
• You can always call your weapon to you, even if it is in different planes or dimensions.
• If your weapon gets destroyed you can perform a ritual for 30 minutes to call it back in perfect condition.
• If your weapon is a ranged weapon you always have ghostly projectiles that you can use as ammunition. If it is a melee weapon your range is increased by 5 feet.

Eternal Binding

With level 18 you achieve absolute mastery of your chosen weapon.  You are now able to create an eternal bond to your weapon and increase its power one last time. Choose one of the following effects. This effect is binding and will not change even if you switch to a different weapon.

Supernatural Senses. You destroy your eyes, how is your decision, and gain supernatural senses. You gain blindsight within 120 feet as well as permanent true sight of 30 feet.

Telekinesis. Your weapon becomes a spell focus and enables you to cast the spell "Telekinesis" without the spellcasting ability or spellslot. Your spellcasting ability is calculated from 8 + your proficiency bonus + your dexterity or strength modifier (your choice).  Your dexterity or strength modifier determines how often you can use the spell before you need a long rest.

Ghostly Weapon Companion. You bind the spirit of an old warrior, for example a samurai or warlord, to your blade and can use an action to call him to your side. The ghost has the stats of a "Sword Wraith Commander", but hast an appearance of your choice. You can use this feature only once per short or long rest.

Demonic Guidance. You bind a creature to your blade, this can be a demon or other otherwordly being. This gives you unbelievable precision, almost as if your weapon is guided by supernatural powers. Whenever you make an attack you can sacrifice 1d10 hit points and add that value to your throw. You can do this even after the DM has given your result, the weapon then basically magically strikes at the target almost by itself.