How does this homebrew Warlock “Patron of the Old Blood” compare with official patrons

No, it is not balanced

Warlock sub classes tend to be quite powerful, as a rule. So it's understandable you would want to give your class some major benefits. However, there are several balance problems with this class.

Access to overleveled spells

Two of your four class features above are dedicated to the daily casting of a spell. Given Warlock's limited abilities to cast spells above 5th level, this is a reasonable idea for a class feature. However, in both cases you've given the warlock access to a spell several levels before another caster would get them.

A full caster (e.g. Wizard) would normally have access to 7th level spells at 13th level, and 8th level spells at 15th level. But you've given this class access to the 7th level spell Plane Shift at 10th level, and the 8th level spell Maze at 14th level.

The spells in this game are balanced around the assumption that characters get access to them at the appropriate levels. In fact, your version of Plane Shift is considerably more powerful than the usual version, since you do not require a different 250 gp focus for each plane. So it's likely that these earlier access to superior spells might cause some balance issues in play.

I would recommend allowing the warlock to cast Plane Shift as described as its 14th level feature, and replace the 10th level feature with something else. That might be more balanced compared to other classes.

Resource-free healing

As it is currently worded, your "severance" feature ensures that after any combat, you can quickly heal yourself up to half hit points without spending any resources (other than bonus actions), unless you've lost a limb. This is an extremely powerful ability, that essentially means you will always be at half or higher hit points after every combat.

The ability to regain a limb is also quite powerful. Although many DMs don't include the possibility of limb loss, it is a major impediment to a character if it happens. The lowest cost way to heal it normally is the 7th level spell Regeneration, which usually could only be cast at 13th level. To give a character access to this at 6th level is very likely to cause balance issues.

If the ability to regain 1d4+Cha hit points was limited to once a short rest, or took an action and only gave you temporary hit points, and the ability to reattach a limb was removed until 14th level, this feature might be more balanced.

Your expanded spell list may be overpowered

Haste is one of the best buff spells in the game. The ability to cast it twice per short rest at 5th level may easily be unbalancing on its own. And the ability to cast Death Ward multiple times throughout the day (essentially spending any unspent spell slots on it for another party member whenever you are about to take a short rest) is a serious balance issue on its own. It might be ok to have one of these spells on your expanded spell list (although either would be a campaign defining class feature), but I'd heavily recommend against having them both.

Serenity's Shadow is problematic when combined with other characters

The ability to have an "always on" advantage to stealth checks is quite powerful to begin with. Add to that a +1 or +2 to AC at all levels, and you've got a pretty sizable advantage over some other classes. But these features in and of themselves are unlikely to unbalance your character, and is more or less in line with what other warlock classes get at first level (especially front loaded ones like the Hexblade).

These features may cause some issues when multiclassing (Rogues and Paladins would both jump at the opportunity to have these "always on" abilities). However, that isn't necessarily out of balance with established classes either (again, compared to the Hexblade).

What makes Serenity's Shadow problematic is the ability to inflict vulnerability to "slashing, bludgeoning, and piercing damage until the end of your turn." Compare this to the Grave Cleric's chanel divinity ability: this gives vulnerability to one attack, and it costs the Cleric's action to use. Your ability will work for an unlimited number of attacks, and lets the warlock do a normal attack in order to use it.

As a simple example of how this could be abused, consider a team with a Necromancer Wizard, a Rogue, and your Lady of Pain Warlock. The Wizard could instruct all of its skelletons to use the Ready action and fire at your enemy when you attack it. Similarly, the Rogue could Ready an action to fire with the same trigger. All of these attacks would then deal double damage to the target (since the imposition of vulnerability has no saving throw and does not require the Warlock's attack to hit), essentially giving most of your team a free turn due to your one ability.

This ability wouldn't be game breaking for a lot of parties. After all, at higher levels most martial characters (who are the ones who usually deal the most bludgeoning, piercing, and slashing damage) do more damage on their own turns than double the damage of one attack, so it would be a mistake for them to Ready actions and attack once on your turn. But this ability is potentially encounter ending enough that an entire party might design their characters based entirely around it. I would not be at all surprised to find this happening.

Don't give up! You're off to an intriguing start

A lot of your proposed features are extremely interesting. I especially like how you have them all tied around coherent central themes. But I think most if not all of the features proposed here need some rebalancing before it's a fully balanced Patron.

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.