Drow Matron Mother (MToF)
The Matron Mother is the mortal boss of the Drow, representing the height of political power and divine favor (or what passes for either in their society). They are Legendary CR 20 Clerics, with a full suite of Lair Actions. Despite this, the book goes out of its way to say you really shouldn’t run this creature by themselves. In fact, it specifies that you should use the other Drow in this book, and any other underdark creature you want, to beef up the encounter. The book also reiterates that Lolth is a fickle goddess, and that the power granted to a Matron Mother can be taken away. Allowing you to set up an alternate win condition for the encounter. Here is a possible example: “See that giant Lolth statue in the throne room? Would be a crying shame should anything happen to it, the entire House would lose favor!”
The next paragraph goes on to discuss how to make a Matron Mother who denounced Lolth, which is just deleting their ability to summon demons, and their abilities that boost their allies. Quite frankly, this is an underwhelming option, those powers could have been replaced with the ability to summon anything else, and a more generic ally buffing power. If you aren’t having them summon fiends (and realistically, they still can if they hook up with another evil god). You could have them summon a Deva (From the MM and the Basic Rules) or maybe even the fey Eladrin (later in this book) to represent getting back on the good elf team. Even Slaadi are available to fill in the role of Aberrations. There is, however, a distinct lack of CR 10 undead to summon. Fickle Favor could be replaced with something that just grants advantage without the damage.
Aside from that, it’s relatively easy to refluff the Matron into a different endgame Cleric threat, should you need one. The only other thing you would have to consider is their lair actions.
Speaking of Lair Actions, this book has them for the Matron Mother, which is a nice addition considering they weren’t in MToF. Perceive Interlopers allows them to negate invisibility, and even mundane hiding (which is sure to put a pinch on your high level Rogues). Spectral Web is a basic AoE restrain without any of the limitations or side effects of the Web Spell. Telekinetic Throw is a rare STR save that can toss people around, even up into the air! For maximum damage: smash someone into the ceiling then let them drop back into the ground. Or just use it to reposition the players as you see fit. It’s a fun way to throw a temper tantrum for sure.
For an out of combat fun, the Matron Mother can cast Clairvoyance, allowing for some magical spying in addition to all the normal spying they will be having their underlings doing.
Back to combat, the Matron Mother has a multiattack that can be used to attack up to 4 times if you want to use the Tentacle Rod, and you should really be using the Tentacle Rod. Not only does it have superior range, it has massive control options, limiting the target’s speed, preventing reactions, and even making it so they can’t use both an action and a bonus action in the same turn. Once everyone has been hit with the Rod, you can switch to hitting everyone with the Demon Staff to inflict fear on them, for even more control. You can even use the Staff as a Legendary Action should you have any left over to burn.
The alternative to multiattack is using Divine Flame, and let me tell you, while it is fun to smite your enemies with a 28 damage AoE, you should stick to the Rod and let your minions kill everyone instead.
You don’t even have to pick between multiatack and using a spell, because using a spell is a legendary action, and boy do you have doozies some spells to pick from. Gate is an instant get out of jail free card that can be used to hall in an entire army, not just any kind of army, but an army from a different plane. All the demons you need to bolster the encounter should there be an ambush. Or just one specific high CR threat, depending on your DMing needs. After the Gate is used up, consider using Banishment to get rid of a high-threat enemy, or Blade Barrier to divide up the battlefield. Should the need to escape arise, there is always Plane Shift. And Dispell Magic is always handy against other casters.
Unlike other support units in the book, the Matron Mother has access to the Cure Wounds spell. But can only cast it at level 1 power due to monster spellcasting mechanics. This problem was mentioned upthread when we were discussing bards. Though I do believe it could be mitigated by just having an x/day healing power. But it’s nice to see that healing still exists for monsters.
As if having legendary actions and Muiltiattack wasn’t enough, the Matron Mother also has bonus actions: The ability to summon a Glabrezu or Yoghlol. Of the two, the Glabrezu is generally better for control, and the Yoghlol has more damage along with the ability to ignore webbing that may be all about the room. The Matron also has the Compel Demon legendary Action to get some extra attacks out of their demonic allies.
Lolth’s Fickle Favor is something that hurts and helps an ally at the same time, dealing damage in order to get advantage on an attack roll. It’s flavorful and malicious because it emphasizes the Matron’s total disregard for her servants.
Needless to say at this point, the Matron Mother lost so many spells in the changeover. From 39 down to 18, notably losing Freedom of Movement, Spiritual Weapon, Death Ward, and Guardian of Faith. They also lost the ability to summon a Retriever (despite the fact that the Retriever exists in this book) gaining the ability to summon a Glabrezu in its stead, and the ability to use this summon as a bonus action. The changes to the spellcasting mechanic prevent upcasting, as noted with Cure Wounds above. But in return they can use any spell as a legendary action.