For starters, you can put this in either Ravenloft or in Mabar, depending on where you want to run it. So that's nice. And that means you can put other Ravenloft domains in Mabar as well, if you want to.
Metrol is a city that has been under constant siege for four years, and it shows. So many things have been conscripted for use by the state. Stores are practically empty, with no hope of being restocked. Gold and silver coins are basically worthless, while copper is in high demand for weapons-making. You get ration coupons to obtain food (which is always Metrol ragout; don't ask what's in it), and you're incredibly limited in what you can buy. There's basically nothing new in Metrol. If someone were to smuggle in food or clothes, they'd make off like bandits... assuming the government didn't claim it all for themselves.
Metrol is attacked every night by "Karrnath" (clearly creations of the Mists), similar to how Falkovnia is attacked on each month on the new moon by undead. Except that in Metrol, the attacks are different each time and are totally unpredictable, both in how they they are waged and what they involve. One night, it might be a a platoon of seige weapons; another night, a curse-bomb; a third night, a dracolich. Because of this, there is a constant need for soldiers, so press-gangs abound.
Despite, or because of, the constant threat from outside, an enormous amount of paranoia and distrust has arisen on the inside. People are blaming anyone they can think of for their problems, ranging from racial intolerances (halflings and changelings are untrustworthy criminals) to religious intolerances (the Church of the Silver Flame is somehow responsible). And anybody can be a Karrnathi agent.
One entire section of the city has been cut off and is now a no-man's land, filled with undead. There's also a huge undercity. This undercity hasn't suffered from the Karrnathi bombardment, but does have its own problems with plagues and curses, and the occasional undead. There's also the area outside the city--but some of it has been mined by the Karrnathi.
Queen Dannel is the Darklord. She's convinced that she will win this war, if she just sacrifices enough to do so--and if only it weren't for the betrayals and incompetence of those around her. She can create amazing constructs (a gift of the Dark Powers) but has no understanding of the Mists and can't open her domain's borders. She has a full statblock; she's CR 15.
There's a section on how the various Dragonmarked houses are doing in Metrol. House Cannith is making cyborgs and flesh constructs because they other resources (this is an excellant reason to play a reborn from VGR). House Ghallanda runs the Ragout Kitchens (seriously, don't ask what's in it; there's a table). While they can make good food via magic, that food goes to the queen and her advisors and other important people. House Jarasco has taken to using questionable methods to keep people alive, including turning people into golems. House Lyrandar has a weather-control beacon, although they can't banish the Mists. House Vadalis is creating magebred warbeasts and lycanthropes. Other hosues have minimal presence at best.
Other organizations include a Fifth Column, which may or may not actually exist, but people sure think it does; the Queen's Watch, which watches out for treachery; and the Unbroken, who seek to protect Metrol from the horrors within (such as the Queen's Watch).
There's a big section on making adventures in Dread Metrol, including a fun little mix-and-match random adventure generator and several fleshed-out hooks. It also includes info on incorporating dark gifts (from VGR) and fun ways to alter your class, race, and background to reflect the fact you've lived in a siege for the past four years. For instance, if you were made by the joint Vadalis/Jorasco attempt to create people with animalistic/monstrous parts, you can justify having all sorts of monstrous traits.
There's a "reconstructed" template that you can put on people (or animals, or monsters, or a PC) to make them effectively half-warforged. It's basically a new lineage, although it's not described as such.
There's a new artificer archetype: the Mastermaker, where you can make cyborgs. At 3rd level, you can make and animate an apparently unlimited number of prosthetics (they don't have special abilities). You can replace one of your own arms with a Battlefist with which you can attack--using Int for to-hit and damage modifiers. At 5th level, you get Extra Attack. At 9th level, you can use infusions and special abilities on your Battlefist. At 15th level, basically become as much of a construct as you want to be (seriously; you can choose to count as either a humanoid or as a construct when you're affected by a spell or magical effect.)
There's a big adventure I haven't read yet.
The monsters are mostly reprints of existing ones, for use in the adventure. There's a few new ones (or new to me, though), though: burster bugs, which implant larva; deadwings, which I imagine are undead stirges; spider-horses, which do whatever a spider-horse does; shredderfish (magebred quippers); wargoyles; and vulgres. Strangely, there's no description or lore on any of these monsters. I have no idea what a vulgre looks like. The only artwork, which is in the adventure, is for something that's clearly a hook horror. The statblock for the vulgre doesn't include hook attacks, though.
The rest of the book is maps.