How did Mercendele Died? Please describe with details. I get it.
The details don't matter. If you want them, read the book. Patrick Weekes put a lot of effort into writing it in a particular way to achieve a specific effect. Just rattling off the details completely ruins the point.
This is basically like asking someone to describe the special effects in a movie to you. "Ooh, there were explosions?! What color were they?!" it doesn't matter. If you want to know, see the movie.
Everything impactful that happens, story-wise, was synopsized in Asunder--apart from the bit about the Eluvians. Celene and Gaspard are fighting for the Orlesian Throne. Celene requests Justinia to make some sort of move to stabilize the mage/templar situation (Asunder details how that works out). There is an elven rebellion in Halamshiral that Celene puts down. The Masked Empire is not a new step in the story, it's a character study of Celene, Gaspard, and some of the important people who surround them that establishes the nature of their personalities and relationship a little better.
If you want some character details, here they are:
Celene cares about the arts and sciences. She prefers to reign over Orlais by subtlety and misdirection, and is trained as a bard. She's in love (and lovers) with Briala, her elven handmaiden, but Celene is also ruthless enough to have Briala's parents killed and to viciously put down an elven rebellion to maintain her authority. When Briala finds out about this, it doesn't go over very well. At the end of the book Briala abandons Celene to forward the elven cause by herself.
Briala is a more passive character--she tries to get Celene to help the elves. She's trained as a bard as well and acts as an assassin and spy at times to assist Celene. After her parents died she fled to the Dalish and met with Felassan (an elven mage who seems Dalish but there's some kind of mystery about him). He taught her and sent her back to Celene. Briala turns on Celene (kind of) at the end and acquires the pass phrase for the Eluvians, which she plans on using to help liberate the elves.
Gaspard is a chevalier. He sees everything through the lens of the military and isn't terribly subtle. He's strictly honorable but, as he says repeatedly throughout the book, honor doesn't preclude tactics. He's not above stabbing people in the back and taking advantage of their weakness, but he won't torture and he keeps his given word.
Michel is another chevalier and Celene's champion. He's even more honorable than Gaspard. However, he's also a half-elven commoner, a fact that was concealed. Gaspard hired a bard to dig into Michel's past as part of a strategem and Celene sends Briala to rescue him, so Briala finds out about Michel's concealed past and uses it somewhat later to get quits from both Gaspard and Celene.
Felassan is a Dalish elf mage (maybe). He's bitter and world-weary and there are some hints that he's been around for a long time, knows far more than he says, and possibly worships Fen'harel as his patron. He assists Briala but the reasons are never clear. He also seems to know something about the demon Imshael and what that demon is after. Felassan is also a somniari. Here's a lore tidbit: according to Felassan the ancient elves who entered uthenara had some kind of power over the realm of dreams, but what is not specified. It's possible that, as a somniari, though, Felassan still has some contact with any of them that remain "alive". Apparently once the sleepers reach "perfection" they can draw sustenance from the Fade and stay "alive" basically forever. So it's nominally possible there are some still-living elven dreamers in some long-forgotten tomb somewhere, although it appears that when the Eluvians were shut down and the elven empire fell the servant class killed the sleepers they could reach, so maybe not. There's a small indication that Felassan has some advanced warning of the whole Veil situation coming, but zero details. A mysterious shadowy figure kills Felassan on the last page.
Imshael is a demon (although he refers to himself as a "choice spirit") that was summoned by the Keeper of a Dalish clan to awaken the Eluvians, but Imshael found the Dalish too tepid and uninteresting, so he finagles Michel into freeing him in exchange for opening the Eluvians. Imshael also murders the Dalish clan that summoned him, all except for the Keeper's First, who he possesses. If Imshael's word is worth anything he feeds off, basically, drama and conflict, so he wants as much drama and conflict around him as possible.
Those are the major players. Everyone else is a bit part at best. Mishra, the First of the Dalish clan, is the only other person who survives, but there's no indication that she has an important role going forward. She may show up, who knows.