You summoned me! Alright lets take a look at this guy.
Lets start with my biggest issue with the original Mummy Lord.... durability. The guy was so darn weak that my party killed a mummy lord in 1 round on two separate occasions. Does this one hold up?
Its clearly got some better adjustments. More hitpoints and a slightly lower CR, some regeneration, and more status effects to keep the party's offense down. And yet....ultimately I'm calculating a defensive CR of 9... that's still just too low. A 12th is level party is still going to kick the crap out of this thing in short order.
I think we can play around with the Regeneration here, lets make this thing really freaking cool. Like regen 20 or 30, and say that in order to stop the regen you have to crit the creature with a slashing weapon (exposing its heart), and then pour holy water on the heart to stop the regen for a round. Now you have a really interesting challenge for the party, and a very memorable encounter. I think that is often missing from legendary creatures....its not just legendary power....its legendary weirdness....killing them isn't just about about applying damage to hitpoints, you have to do something special to really fit the bill.
Alright so now on to offense:
The Negative Energy aura is a nice touch, adds some good fear factor. The kopesh is worded strangely for the crit. So is it the same necrotic damage added to the kopesh, or is it all slashing now.... its a bit unclear. Also, I'm not sure the Kopesh is really needed, I mean the rotting fist gives mummy rot, and with two you grapple and can drain max life.... I'm not really sure what having the kopesh really brings to the table, personally I think you could remove it to make the statblock tighter and lose nothing.
Dreadful Glare: So this ability is actually really really bad. Frightered is a weak effect for consuming your action, and the DC is far too low to consider the paralysis a thing, especially with the very short duration. the ONLY reason the Orig ML can work with it, is you can use the effect as a legendary action, so its cost was lower. Without that, I would never use this ability in combat. I would either drop it entirely, or change it to all creatures within X, that way it could possibly have some punch.
Animate Dead: A nice add in to common mummy flavor.
Death Ray: Honestly nothing really exciting here, its just some range damage with nothing really all that special, and doesn't have the same cool flavor as some others. I would drop this one.
Desert Petrification: This is the coolest addition in the statblock. The idea of a mummy turning someone to sand is just horrendously awesome! My one change here, it should be constitution...I just can't accept the flavor of a dex save here it just doesn't make sense to me.
Putrid Cloud: The effect is pretty useful, but instead of gas I would go with a swarm or something, something biblical

I think you could do a straight flavor swap and keep the effect.
Sandstorm Breath: It works I guess, I don't know something about just doesn't give with me. If nothing else, I don't see a good reason to go with both force and slashing, I think just pick one or the other.
Blasphemous Word: If your going to keep this, I would give the mummy some kind of life sense, aka that they can detect hp. This allows them to target creatures more exactly, and I think a life sense concept makes good sense for a mummy.
Dreadful Curse: Frankly I think the desert petrification is so much neater, I wouldn't mind this ability just being "recharge and use desert petrification again". between the petrification and the mummy rot, I think you already have enough cursing going on.