What did I do wrong? Managing a Mummy lord to be an actual challenge. /u/CrotodeTraje DnD: Roll for Initiative!

A couple weeks ago, as I was thinking in which direction to take my party on, I asked in this subreddit and I was told an Adult Black Dragon (CR 14) was too much for a 4-person party of level 8, and that I was going to end my campaign in a TPK for sure.

I finally decided on a mummy lord, read on all his abilities, and decided both to pump up his HP and give it two regular Mummies as “tanks”.

I did This, not because I’m evil, or I know so well D&D maths, but because after so much time with my party, I have come kind of “extra sense” for how hard can things get. But I’m oblivious as to why does it actually works.

As I played, I noticed I did make some mistakes (regarding strategy) but I was wondering if this sub couls help me identify other mistakes that may have caused the enemy to not be so efficient or so lethal.

1. The players win initiative, the mummies goes at the middle, the mummy lord is the last one to play and is located in the other end of the tomb (the regular mummies being much more neat the entrance).
2. The players enter the tomb. The mummies and the Mummy Lord are inactive yet. No Lair actions.
3. The mummies only “awake” after the character touch the sarcophagus. the two mummies deliberately don’t use their Dreadful Glare in the first turn. One of the mummies soak up a big deal of damage.
4. The mummy lord awakes, Uses one cantrip (sacred flame – I increased damage to 3d8) and upcasted Spiritual Weapon with a 5th level slot.
5. Before the end of the first round, the Mummy lord uses 2 legendary actions to move by surprise to the weakest character, and then on another turn, make one attack.
6. Beggining in the second turn, the mummies start making use of their multi attack. the two targeted PCs pass their Con check to avoid being frightned.
7. After all the PCs have played, the Mummy Lord splits his attacks. Targets one character with Dreadful Glare (saves), another character with Spiritual Weapon, and makes one attack with Rotting Fist against the weakest one.
8. After another round of combat, the Mummy lord uses the last two turns of the PCs to make one attack and use Blasphemous Word, but because of initiative order, Players argue that they aren’t stunned at the beggining of their turn, as the Mummy makes use of his turn, before they do. bad timing by my part, I have to agree with the players. In the turn of the Mummy Lord, casts harm on the Barbarian that had targeted the Mummy Lord with Ancestral Protectors. The mummy lord deals some big dice of damage, Barbarian saves the CON check, takes only half damage (because its necrotic, it doesn’t get split in half), is left in 1HP.
9. At some point, in another round, the Mummy lord gets to use Blinding Dust: while being fully surounded, blinds for one turn 2 of the melee PCs.
10. The Paladin missed a lot of his attacks (lets say, the first 3 or 4 turns), but when it landed, used Divine Smite to great effect.
11. One of the regular mummies falls to the fighter, and he approaches the Mummy lord (still not at melee), the Paladin is dealing with the other one, finally decide to ignore the regular mummy and go deal with the Mummy Lord. Mummy Lord gets surrounded by 3 players.
12. Rotting fist + Dreadful Glare, the Bard gets paralized. The barbarian falls.
13. The players get the barbarian up again, The fighter makes a complete attack, use battle maneuver, then Action surge all again. I think there was a critical hit somewhere. The Mummy lord expend all his original HP, but I keep them alive to make the fight more intense.
14. The Bard gets close to death, the Paladin is half way through, the fighter is spotless. The mummy lord uses the last PC turn before his, to use legendary action Whirlwind of Sand and get to the other corner without causing opportunity attacks. On his turn he casts insect plague, targets the whole party + left over Mummy. The Barbarian gets again to 0, all the characters suffer damage. The Paladin heals the barbarian and eds his turn outside the area. Barbarian gets up and ends his turn outside the area. Fighter gets closer to the mummy, but not at melee. The bard gets outside the area and casts fireball.
15. the Mummy Lord was already above his origial HP. Bard rolls 28 fire damage. Mummy lord is vulnerable, receives 56 fire damage total.

Although yes, the mummy Lord was deffinitely a hard challenge, not near deadly nor TPK level. I’m sure is all about me not using the mummy lord to its full potential, making mistakes or not playing tactically enough. I want advice as to what could have been a better wat to play the monster as intended, to accurately represent his real challenge rating.

A couple weeks ago, as I was thinking in which direction to take my party on, I asked in this subreddit and I was told an Adult Black Dragon (CR 14) was too much for a 4-person party of level 8, and that I was going to end my campaign in a TPK for sure.

I finally decided on a mummy lord, read on all his abilities, and decided both to pump up his HP and give it two regular Mummies as “tanks”.

I did This, not because I’m evil, or I know so well D&D maths, but because after so much time with my party, I have come kind of “extra sense” for how hard can things get. But I’m oblivious as to why does it actually works.

As I played, I noticed I did make some mistakes (regarding strategy) but I was wondering if this sub couls help me identify other mistakes that may have caused the enemy to not be so efficient or so lethal.

1. The players win initiative, the mummies goes at the middle, the mummy lord is the last one to play and is located in the other end of the tomb (the regular mummies being much more neat the entrance).
2. The players enter the tomb. The mummies and the Mummy Lord are inactive yet. No Lair actions.
3. The mummies only “awake” after the character touch the sarcophagus. the two mummies deliberately don’t use their Dreadful Glare in the first turn. One of the mummies soak up a big deal of damage.
4. The mummy lord awakes, Uses one cantrip (sacred flame – I increased damage to 3d8) and upcasted Spiritual Weapon with a 5th level slot.
5. Before the end of the first round, the Mummy lord uses 2 legendary actions to move by surprise to the weakest character, and then on another turn, make one attack.
6. Beggining in the second turn, the mummies start making use of their multi attack. the two targeted PCs pass their Con check to avoid being frightned.
7. After all the PCs have played, the Mummy Lord splits his attacks. Targets one character with Dreadful Glare (saves), another character with Spiritual Weapon, and makes one attack with Rotting Fist against the weakest one.
8. After another round of combat, the Mummy lord uses the last two turns of the PCs to make one attack and use Blasphemous Word, but because of initiative order, Players argue that they aren’t stunned at the beggining of their turn, as the Mummy makes use of his turn, before they do. bad timing by my part, I have to agree with the players. In the turn of the Mummy Lord, casts harm on the Barbarian that had targeted the Mummy Lord with Ancestral Protectors. The mummy lord deals some big dice of damage, Barbarian saves the CON check, takes only half damage (because its necrotic, it doesn’t get split in half), is left in 1HP.
9. At some point, in another round, the Mummy lord gets to use Blinding Dust: while being fully surounded, blinds for one turn 2 of the melee PCs.
10. The Paladin missed a lot of his attacks (lets say, the first 3 or 4 turns), but when it landed, used Divine Smite to great effect.
11. One of the regular mummies falls to the fighter, and he approaches the Mummy lord (still not at melee), the Paladin is dealing with the other one, finally decide to ignore the regular mummy and go deal with the Mummy Lord. Mummy Lord gets surrounded by 3 players.
12. Rotting fist + Dreadful Glare, the Bard gets paralized. The barbarian falls.
13. The players get the barbarian up again, The fighter makes a complete attack, use battle maneuver, then Action surge all again. I think there was a critical hit somewhere. The Mummy lord expend all his original HP, but I keep them alive to make the fight more intense.
14. The Bard gets close to death, the Paladin is half way through, the fighter is spotless. The mummy lord uses the last PC turn before his, to use legendary action Whirlwind of Sand and get to the other corner without causing opportunity attacks. On his turn he casts insect plague, targets the whole party + left over Mummy. The Barbarian gets again to 0, all the characters suffer damage. The Paladin heals the barbarian and eds his turn outside the area. Barbarian gets up and ends his turn outside the area. Fighter gets closer to the mummy, but not at melee. The bard gets outside the area and casts fireball.
15. the Mummy Lord was already above his origial HP. Bard rolls 28 fire damage. Mummy lord is vulnerable, receives 56 fire damage total.

Although yes, the mummy Lord was deffinitely a hard challenge, not near deadly nor TPK level. I’m sure is all about me not using the mummy lord to its full potential, making mistakes or not playing tactically enough. I want advice as to what could have been a better wat to play the monster as intended, to accurately represent his real challenge rating.