[TRIGGER WARNING: SELF-HARM]

Hello everyone! I've just come up with a quick draft of a subclass for monks. It's my take on the self-flagellation rites that people undertook in medieval times. I was wondering if it was too overpowered or underpowered, so I could fix it. Here's my draft I've made: 

WAY OF THE FLAGELLANT

The flagellant’s path is decried by its opposers as a fanatical, needlessly masochistic path. Those who follow such a path realise its benefits; by purging the body, you also purge the soul. Flagellants have occasionally been employed as torturers by churches, but many choose to wander the land as a vagabond.

Tool of the Flagellant - 3rd-level Way of the Flagellant feature

You gain proficiency with the whip if you don’t already have it. The whip is a monk weapon for you.

Additionally, you can target yourself when you attack with a whip. If you do, the attack automatically hits.

Flagellation - 3rd-level Way of the Flagellant feature

As a novice flagellant, you have learnt the basics of of the whip. When you take the Flurry of Blows ki action, you can replace one of your unarmed strikes that constitute this action with an attack from a whip.

Additionally, when you hit yourself with a whip attack, you can choose from one of the following benefits:

• Restore the Soul: You regain a number of ki points equal to one roll of your martial arts die.
• Bolster the Soul: You gain a number of temporary hit points equal to two rolls of your martial arts die.

Once you have used one of these benefits, you cannot use that specific benefit again until you finish a long rest.

Inured to the Scourge - 6th-level Way of the Flagellant feature

You are inured to the pain of the whip, and thus the whip gains more power. You gain the following benefits: 

• You gain resistance to piercing damage.
• Your attacks with your whip count as magical for the purpose of overcoming resistance and immunity to nonmagical attacks and damage.
• You can use your Constitution modifier, instead of your Charisma modifier, to make an Intimidation check by attacking yourself with a whip.

Mortify the Flesh - 11th-level (Subclass) feature

Your mastery of flagellation improves, as does the strength of your spirit. You gain the following ki features:

• Painful Lash: When you hit another creature with a whip attack, you can spend 2 ki points to make the target bleed. The target must make a Constitution saving throw. If they fail, they take additional piercing damage equal to 1 roll of your damage die, and they also take 1d6 piercing damage at the start of each of their turns for 5 rounds. If the target succeeds the saving throw, they take half as much of the initial damage and do not take the persistent damage.
• Spiritual Fervour: When you hit yourself with a whip attack, you can spend 4 ki points to whip yourself into a spiritual fervour. While in this fervour, you gain a +2 bonus to AC, you have advantage on Dexterity saving throws, and you gain an additional action on each of its turns. That action can be used only to take the Attack (one weapon attack only), Dash, Disengage, Hide, or Use an Object action. This fervour lasts for 1 minute or until you are incapacitated.

Rapturous Pain - 17th-level (Subclass) feature

Your resistance to pain has turned to a twisted delight. When you hit another creature with a whip attack, you ignore that creature’s resistance to piercing damage. If the creature is immune to piercing damage, they still take half damage from your whip.

Additionally, when you hit yourself with a whip attack, you immediately heal a number of hit points equal to the damage you dealt. If the damage would bring you to or below 0 hit points, you do not heal in this way.

My plan was for the monk to draw benefits from using a whip, as well as from self-flagellation. Have I gone too far? Would this work as a subclass? Any helpful feedback will be appreciated!

>When you take the Flurry of Blows ki action, you can replace one of your unarmed strikes that constitute this action with an attack from a whip.

You should make this just all of them. The only advantage of a whip vs unarmed strike is reach.

>Bolster the Soul: You gain a number of temporary hit points equal to two rolls of your martial arts die.

This is pointless, you'll be damaging yourself for as much or more hit points that you regain as temporary hp

>If the creature is immune to piercing damage, they still take half damage from your whip.

This one is also pointless, as far as I'm aware there aren't any creature immune to piercing damage

>Painful Lash: When you hit another creature with a whip attack, you can spend 2 ki points to make the target bleed. The target must make a Constitution saving throw. If they fail, they take additional piercing damage equal to 1 roll of your damage die, and they also take 1d6 piercing damage at the start of each of their turns for 5 rounds. If the target succeeds the saving throw, they take half as much of the initial damage and do not take the persistent damage.

This is a bad feature, it is more expensive than Stunning Strike and is less powerful than Stunning Strike.

In general, it's a bad idea to make a subclass restricted to only a single weapon type because it greatly limits the creative potential and flexibility of that character. What if the party find a really awesome magical shortsword? If this monk wants to use it they have to give up their entire subclass to do so.

What Agilemind said, but also whips do slashing damage, not piercing.