I have a party of 4, level 13 players. One is a Paladin who has an aura that prevents being frightened as well as a +1 Sword. Two characters can do fire damage. The rest of the party is a Druid, Sorcerer, and Bard. They wipe out solo monsters quickly; and use Banishment. The Paladin did about 80 points of damage in one attack on a nat. 20, and two of the party members can cast fire-based attacks. I think the Mummy Lord wouldn't last more than 2 rounds solo.
I'm thinking of giving the Mummy gold-plate armor that is fused to the mummy's body that grants fire resistance (removing the fire vulnerability), but adds vulnerability to lightning (gold is conductive) to counter meta-gaming knowledge. That might let it last a couple more rounds.
Adding more monsters to this fight seems best, but not sure how many to add or what type Thematically I was considering regular mummies, an Air Elemental or a couple undead constrictor snakes (looking like Bone Naga but without the lore). Would 2-4 more mummies do it, or is that overkill?
FIRE
Environmentally, you could have natron salts or sand or even baking soda pouring into the room (possibly as a Lair Action) which douse any ongoing fires, including things like
flaming sphere or
wall of fire. This is an environmental solution that suggests certain things about the scene.
There might be a curse that's activated upon entering the chamber or upon dealing fire damage: If you deal fire damage to Mummy Lord, the Mummy Rot curse accelerates, such that anyone afflicted by it suffers the effects faster each round (i.e. 24 hours > 24 minutes > 24 rounds > 2.4 seconds or every round > every turn). This is a "yes, but" solution, "yes you can use fire, but there is a consequence each time you do."
Also remember that against a party packing lots of fire magic, the
mummy lord should make frequent use of its third lair action forcing a CON save from anyone casting a spell of 4th level or lower, or the spell gets canceled and they take some damage. You could take that a step further and add "...unless they bearer has the holy symbol of so-and-so, spellcasters casting spells dealing fire or radiant damage suffer disadvantage on this CON save." This is solution tying the story of your mummy & dungeon in, and making it relevant to the players who are invested and paying attention.
MINIONS
For minions – or as inspiration for your Mummy Lord – in my Khemti home game I have a certain variety of mummy known as the “favored of <god name>” which have maximum hit points (90), the Spell Reflection trait (narratively these are protective runes written on their funerary wrappings), and usually some kind of magical item. These “favored” mummies typically have CR 4 (1,100 XP). Alternately, you could just adopt the Spell Reflection runes on the wrappings for your mummy lord.
I just used a necrophidius (bone snake construct with hypnotizing powers) in a recent game, that I converted from AD&D. You might find a use for it...
Necrophidius
Large construct, neutral
Armor Class 13
Hit Points 44 (8d10)
Speed 40 ft.
STR DEX CON INT WIS CHA
12 (+1) 16 (+3) 11 (+0) 7 (-2) 12 (+1) 5 (-3)
Skills Perception +5, Stealth +7
Damage Immunities necrotic, poison; piercing and slashing from nonmagical attacks that aren’t adamantine
Condition Immunities charmed, exhaustion, frightened, paralyzed, petrified, poisoned
Senses darkvision 60 ft., passive Perception 15
Languages understands the languages of its creator but can't speak
Challenge 3 (700 XP)
Magic Resistance. The necrophidius has advantage on saving throws against spells and other magical effects.
Magic Weapons. The necrophidius’ weapon attacks are magical.
Singular Purpose. The necrophidius is given a single purpose by its creator, such as guarding a treasure or assassinating a target. When its task is done or cannot be completed, the magical animating the necrophidius ends and it crumbles to dust.
ACTIONS
Paralyzing Bite. Melee Weapon Attack: +5 to hit, reach 10 ft., one target.
Hit: 10 (2d6+3) piercing damage and the target must succeed a DC 11 Wisdom save or be magically paralyzed for 1 minute. At the end of each of the target’s turns, it may repeat the save to end the paralysis early.
Dance of Death. The necrophidius rises up on its tail and sways back and forth. Each creature within 30 feet that can see the necrophidius must succeed a DC 11 Wisdom save or be incapacitated, its speed drops to 0, and it cannot look away from the necrophidius. The creature remains transfixed until the dance stops or the necrophidius is killed. A creature that takes damage while under the influence of this power may make another saving throw to break free.
DurrandDurr said:
Concept is the Mummy Lord was a Queen/Cleric of Light, who was willingly changed into a Mummy Lord after death to protect their sacred legendary weapon from their Shadow enemy for eternity. The transformation destroyed much of the Cleric's mind, and no longer remembers her life--seeing anyone who enters an enemy and can't be reasoned with. So I didn't want to add too many pure evil creatures for story reasons.
The way you describe her, I'm wondering if the players' intent/objective will be to stomp the mummy lord to the curb... or try to redeem her. Because if it's the latter (or it becomes the latter during the course of the combat), then you don't need a super challenging battle, and instead what you need are some countermeasures / ways they can restore the mummy lord's mind, so it becomes a question of "how do we
not kill her, but stay alive long enough to
save her?"
