Mummy Lords and CR

[Trask]

You probably underplayed the Mummy Lord's abilities in order to give the players a chance to survive (there is nothing wrong with that, I'm just mentioning it... It's only logical that he may have underestimated them and not fought them to his full potential), consiously or not.

4 level 6 characters would need only fail a Blasphemous World/Dreadful Glare/Hold Person and then a single rotting fist from the Mummy Lord is enough to devastate their HP. He has a base AC of 17, 19 when using Shielf of Faith, and is immune to a lot of damage types; his vulnerability to fire is something that I would consider metagame if used without being either the default attack pattern of a character (a spellcaster using firebolt as his usual cantrip for example) or after a succesfull Knowlage Religion/Arcana check.

Silence can also lock other spellcasters out of their spells, effectivelly giving the mummy Lord an unfair advantage due to his innate (aka non Verbal) magic such as Rotting Fist/Dreadful Glare/Blinding Dust/Channel Negative Energy and Whirlwind of Sand.

At the same time, if non-casters don't have access to magical weapons (and casters can't cast), the Mummy lord is immune to all dammage since he is immune to non-magical piercing/slashing/bludgeoning damage. This leaves the only way to damage him being improvised (thus, no profficiency bonus) attacks with torches, and with a 17 AC it's not easy to hit someone without a proficiency bonus. Finally, the attack would still do minimal damage, as it's an improvised weapon, dealing 1 fire damage + str mod bludgeoning damage, the latter of wich is mittigated due to imunity.

PS: Obviously, there exist non-verbal spells, or spells that could have a duration that continues, or a sorcerer could cast with Metamagic... There are ways around it. I'm just justifying/explaining how a Mummy Lord played at his True Full Potential can be a very big threat for a party, and is worth his CR, if not more.

If I underplayed the Mummy its more indicative of my tactical skill, or lack thereof, rather than any pulling punches on my part. I definitely didn't try to make it easy for them. I just found that the Mummy was so frail a few rounds in melee would leave it in tatters from Monk flurry of blows and druid magic claws. If the Monk got his stunning fist off that would have been it for the mummy lord. I used almost all his spells actually it was such a long battle. I'll keep in mind though the possibility. Still though I'm having a really hard time imagining a party of 4 level 15s having an issue here, like at all.