Yes, that works as written

The steps all logically work together and the damage does stack up together (just don't forget to add the CHA for each Blast.)

Most likely incredibly overpowered.

I personally do not think this is balanced as it presents significantly more always-on damage than nearly any other class and every player would likely want to do be able to do this.

I like the concept a lot! I definitely don't think it's overpowered -- it might be underpowered, but not excessively so, in my opinion.

I think the main issue with this class (which I mention several times in the below points) is that its most powerful feature is Serenity's Shadow which it gets at 1st level. This has 2 issues: it encourages other classes to take a 1 level dip in this class, rather than sticking with it, and it makes later levels kind of boring, since you don't gain as much from later levels. I'd try to spread the power of Serenity's Shadow over multiple levels and I provide a few suggestions for that below:

• Serenity's Shadow:

  1. You should specify that the radius is centered on yourself.
  2. Can any creature be made Vulnerable, even if they'd otherwise be immune?
  3. How long does the advantage on Stealth last? Since DMs often use one check to cover an extended period of stealth, is it ok if the advantage lasts a long time, or it it meant to only apply for the short period before the shadow retracts?
  4. I worry that the Vulnerability feature is a bit weak, since it only applies to a single attack and then reduces your AC for the entire day. @BlakeSteel points out that increasing its duration could be overpowered if other characters coordinate their actions to exploit it, but I see little issue with allowing it to apply to all of your attacks this turn (if you have Extra Attack, Thirsting Blade, Haste, or Two-Weapon Fighting or another form of Bonus Action attack).
  5. The main concern with this would be that it makes a 1-level dip extremely powerful. Moving that feature to a higher level would solve that problem, and give a nice boost to higher levels for a class that gets most of its best features at 1. So, at 1st level, it would only apply to a single attack, while at Level 10, it could apply to all attacks on your turn.
  6. If you're worried that the change in 4) is too powerful, you could make it kick in only at a higher level, and/or make it shift Vulnerability by one category: so Normal -> Vulnerable, Resistant -> Normal, or Immune -> Resistant
  7. To make this feature a bit clearer, I'd tie the number of uses to casting the shadow, then list the benefits while the shadow is out, and then list the actions that can be taken which use up the shadow. This way, you don't have the phrase "Once the shadow is used in this way, it recedes until you have taken a long rest", which becomes invalidated once you get multiple uses at 10th level.

• Serenity's Shadow - AC Bonus:

  1. Since the AC bonus is worded as a flat bonus to AC, it would stack with the Monk's WIS bonus to AC, the Barbarian's CON bonus to AC, or the AC bonus from Mage Armor. Is this intended? If not, you should mimic the wording of those features and describe it as an alternate way to calculate base AC.
  2. I don't think letting the AC bonus stack is overpowered, but my main worry is that, since the feature is gained at Level 1, it would encourage 1 level dips, just to get the AC bonus. It's really flavorful, and I like getting it at 1st level, but you should find some way to make it scale with level to discourage 1 level dips
  3. Here's one suggestion for scaling: Let the shadow only be active for a limited number of turns per use (perhaps 1 + Warlock level). So, a 1 level dip would give you a free AC bonus for only 1 or 2 turns per day; good, but not overpowered. At level 5, it's probably safe to make it last indefinitely, since that's a significant investment.

• Severance:

  • Is it meant to have unlimited uses (assuming you have hit dice remaining)?
  • The wording on this is unclear. I'd reword it to mimic the Durable feat:

    "you can use an action to roll one Hit Die to regain hit points, the amount of hit points regained is equal to the die roll plus your Charisma modifier"

• Planar Doorman:

  1. The section that states: "You can also immediately identify the plane ..." I assume this refers to the location they're going to plane shift to, right? That's unclear from the wording.
  2. The first clause should mimic the invocations that add spells: "You can cast plane shift once using a warlock spell slot. You can’t do so again until you finish a long rest. If you are traveling to Sigil, this does not use a spell slot or material components".
  3. Does the reaction to counter plane shift also use up the free planeshift? Does it prevent you from tracking people who try to plane shift?
  4. I think it would be cool if, instead of just countering the casting of plane shift, you also had the option to use plane shift to follow that person.

• I thought you were a deader for sure!:

  • Does this ability only apply to yourself?
  • This ability is pretty underpowered, since most DMs don't give their PCs crippling injuries. I'd expand the healing potential by giving you the change to cast Regenerate as a Warlock spell once per day, perhaps with the side effect of it being excruciatingly painful.
• Restrictions and Benefits:
  • As Miniman points out, the ability to navigate the City of Doors listed under "Restrictions and Benefits" looks like fluff or a roleplaying hook, but could actually be extremely mechanically powerful, especially if the campaign takes place in Sigil.
  • No other patrons have mechanical benefits in their description - if you want this to be a specific power that Lady of Pain Warlocks have, you should make it a class feature, perhaps part of Planar Doorman
  • A better option, in my opinion, would be to remove any explicit powers of navigation and, instead, state that the Lady of Pain can, at her discretion or in exchange for a deal/favor, grant this power to assist players in carrying out her will. This puts control of the feature in the DM's hands and reduces the odds that you'll disrupt the campaign by intuitively navigating to the BBEG's secret lair.