Multi-Subclassing (5e Variant Rule)
From D&D Wiki
Chapter 6: Customization Options of the D&D Player's Handbook 5e provides guidance on options to fine-tune your character with the DM's permission to go a step further. This Radical Rule allows your character to maintain the same class, but pickup different Subclass abilities. This Radical rule uses the established rules of multiclassing with additional rules added in to help preserve character balance. Important rules of Multi-Subclassing: -When your character changes subclass that subclass starts over from level 1. -You will not pickup Features that have already been achieved with your other subclass, except for Ability Score Improvement.
Since your character is not changing classes you will not have to worry about meeting various Ability Score Minimums found in Chapter 6: Customization. However, since your character will be training in multiple subclasses your character will require to meet a higher Ability Score Minimum. The new requirement is the Class Ability Score Minimum +2.
Example: Bard Charisma:15, Fighter Strength:15 or Dexterity:15, Ranger Dexterity:15 and Wisdom:15
There are no changes to this rule from the standard Player's Handbook 5e Chapter 6. Your character level will always follow the Character Advancement table in Chapter 1.
Hit points and Hit Dice
Since your character is not changing classes the Hit Points and Hit Dice will be based off of your character's level for that class and follow the standard Class Features.
Example: A Fighter who is of character level 20 and achieved Champion level 5, then chose to change subclass and achieved Battle Master level 15 will still have Hit Dice: 1d10 per character level so 20d10. Hit Points at 1st level 10 + your Constitution modifier Hit Points at Higher Character Level 1d10 (or 6) + your Constitution modifier per character level after 1st.
There are no changes to this rule from the standard Player's Handbook 5e Chapter 6. Proficiency Bonus is always based on your total character level.
There are no changes to this rule from the standard Player's Handbook 5e Chapter 6. You do not gain additional proficiencies when you begin your character archetype over at level 1.
A very important rule of Multi-subclassing is that your class level starts over at level 1. You do not gain any class features previously earned, with the exception of Ability Score Improvement. *NOTE*:Because Multi-Subclassing grants extra features related to an archetype your character will not achieve some higher level class features.
Example: A Rogue of Character level 20 has Multi-Subclassed Thief 4/Assassin 16. This character will not achieve class features above level 16, but will have a Proficiency Bonus +6, Sneak Attack 10d6 (both based off of character level) and the following class features Level 1 Thief: Expertise, Sneak Attack, Thieves' Cant Level 2 Thief: Cunning Action Level 3 Thief: Roguish Archetype: Thief (Fast Hands, Second-Story Work) Level 4 Thief: Ability Score Improvement Level 1 Assassin: No Features granted Level 2 Assassin: No Features granted Level 3 Assassin: Roguish Archetype: Assassin (Bonus Proficiencies: disguise/poisoner kit, Assassinate) Level 4 Assassin: Ability Score Improvement Level 5 Assassin: Uncanny Dodge Level 6 Assassin: Expertise Level 7 Assassin: Evasion Level 8 Assassin: Ability Score Improvement Level 9 Assassin: Roguish Archetype: Assassin (Infiltration Expertise) Level 10 Assassin: Ability Score Improvement Level 11 Assassin: Reliable Talent Level 12 Assassin: Ability Score Improvement Level 13 Assassin: Roguish Archetype: Assassin (Imposter) Level 14 Assassin: Blindsense Level 15 Assassin: Slippery Mind Level 16 Assassin: Ability Score Improvement
There are no changes to this rule from the standard Player's Handbook 5e Chapter 6. Channel Divinity will gain new features based upon your subclass, but it will not grant extra uses. You gain additional uses only when your highest class level explicitly grants them to you.
There are no changes to this rule from the standard Player's Handbook 5e Chapter 6. Extra Attacks are only granted based upon your highest class level.
Unarmored Defense, Martial Arts Damage, Ki Points, Unarmored Movement
Unarmored Defense is granted only once. The benefits are not granted again when starting your subclass over again at level 1. Martial Arts Damage, Ki Points and Unarmored Movement are granted based upon your character level.
There are no changes to this rule from the standard Player's Handbook 5e Chapter 6. Spells known, Cantrips, Cantrip strength, Spell Slots per Spell Level are granted based upon your character level.