Since you are reading this, I will assume you have some knowledge of the Warhammer universe and how Total War games work - otherwise I apologise, because this text will be a lot of gibberish.
The DLC came along with a game update and added a few new things and maps, but let's keep the focus on the three lords. Just like Lord packs before this, these lords don't participate in the main game and its race to reach to bear-god Ursun. They have their very own individual campaigns, but set on the same map. However, new heroes have been made available at roughly the same time, including a Kislevite archer-vampire-spell caster, and the ever-regenerating Aekold Helbrass, while some lords in the DLC get additional legendary heroes such as the flying, but very random Blue Scribe spell caster for Chaos. There are nine new Regiments of Renown and 12 additional units as well. It's pretty good, and it covers some holes in the original roster. Other factions also benefit in general, as the Tzaangor unit is also available to the Beastmen, and so forth.
But let's take a closer look at what you get. The Changeling is the Tzeentchian lord, he is a trickster, meaning that any and all manipulation mechanics are available, but more importantly, he plays a horde style with no fixed settlements, but still has hidden cults just like pirate groves from the second iteration of the game. This is combined with having missions all over the map, and each minor mission giving you an advantage, items or similar, and then main missions that need to be completed. If you complete all of them, the final quest battle is a lot easier. However, I did find it mildly frustrating that you need to keep an eye out for all minor missions at once, and waiting just a few turns meant that I was unable to make contact with a specific lord before he was killed by another faction, thus robbing me of the item he would have rewarded me - a problem that is not unique for The Changeling, and actually caused me to miss out on some, in my view, essential buffs for one of the other Lords.
The Changeling is a vastly different way to play, and I found it very entertaining, and while he seems like a weak spell caster, he is able to turn in to any other lord you have defeated or befriended, and while you only get to choose one, and have to do it before even initiating battle, it means that you can all of a sudden gain flight, regeneration or become extremely tanky as you turn in to the best version of the lords you are imitating. Combined with some new infantry and monster units, the Mutalith Vortex Beast being my favourite, mainly due to its name, You can field an impressively well rounded and versatile army, especially if you buff up your barrier, however, you will still rely heavily on you own spell casting to get the job done.
For Cathay players, there's Yuan Bo, the Jade Dragon, and his mechanics are easy to use and learn, and has the ability to make some serious buffs, like replenishment in foreign territory, +20% movement range in campaign, +10 leadership and +50% ammunition - all at the same time - and if you really did something bad, it can even reset your diplomatic reliability rating to Very High - perhaps one of the strongest buffs campaign wise. He brings a new buffing unit, some really great Jade Lions, and... flying Crowmen... I mean, even for Warhammer, these are a bit too odd for my taste. However, his campaign actually lets you feel under pressure, as you start out being stressed out by Orcs, making the start slightly more challenging than usual.
The last is Kislev's Mother Ostankya, the creepy witch from the woods. She brings a cauldron mechanic to the game with curses and incantations, known as buffs and debuffs, some of them very strong, as well as a new witch hero. She moves around on a missile firing sled, and while most attention has been on her Akshina Ambushers, a hit-n-run style crossbow unit, Kislev has struck gold as she makes her entire army do magical damage - fantastic for killing demons. She also has some truly terrifying monster units, adding speed and a lot of damage to the Kislev roster, making them far better at offensive gameplay, especially combined with the use of giant spider units. The Wood-land-theme is really done well, and I feel the soil between my toes when playing the faction, and as she has it easy allying with other factions as well as getting access to traditional Kislev unites, this roster is extremely versatile, but just more fun when going all out on horrible things lurking in the dark.
And then, the elephant in the room... This DLC is expensive, and its expensive and announced at a point in time where the game was, well, full of bugs to be honest, assets not loading, units not firing or initiating combat, or just plain crashing. It has improved, but it's by no means perfect.
25 Euros is a steep increase for the usual 15-16 Euros for three lords, it's the same price as the entire faction of the Chaos Dwarfs - and honestly, it's a lot of money for some additional content. Granted, I had fun playing it, but 12-15 Euros would already be a very high asking price, and no matter CA's claims of rising costs, I find the pricing disrespectful to the devout fans. It also doesn't take a programmer to recognise that while there are multiple unique lord functions, they are re-remakes of already existing game-mechanics of either other lords or entire factions, so its not like they had to re-invent the wheel. I don't mind paying for good quality extra content, but 25 Euros for three lords and a few extra new units, that is just inexcusable.