Ni no Kuni II: Revenant Kingdom - The Lair of the Lost Lord Review (gsimenas)
TL;DR Only buy if you like grinding or you want to 100% your achievements.
What's included in the DLC:
1. The Labyrinth;
2. Two new fighting styles (Martial Methods);
3. One or two side stories with some actual character building.
4. A few quests related to the Labyrinth or needed to unlock the Martial Methods.
The Labyrinth is basically a 100 floor version of the Faraway Forest (end game Dreamer's Door dungeon) from the base game. Each floor is randomly generated from a limited number of room templates that are randomly connected to each other (and on one occasion, the floor generator messed up and made me get stuck in a dead end). Every ten floors you get a mid-boss fight that nets you a random piece of top-grade loot (if you've collected enough orbs along the way). Unlike the base game Dreamer's Doors, there's no Danger Levels, so there's no time pressure to beat the dungeon quickly; instead you get optional objectives for each floor that give extra orbs. Like the base game dungeons, you can't save while inside the Labyrinth, so you lose all your progress if you die inside, but in this particular dungeon, every twenty floors you can unlock a shortcut to start your next run deeper. The dungeon is nice for some quick levelling and some powerful loot (both in terms of stats, and in terms of having far more than two effects per equipement piece) that you'll need for the other DLC... and that's about it. The "story" of the DLC is paper-thin just to give an excuse for the Labyrinth's existance; there's no character development, nor a real engaging plot to be found here.
Martial Methods is the more interesting part of the DLC, as it adds some new spice to the combat system. This DLC has two Martial Methods: Ding Dong Discipline and Gizmo Supremo. Ding Dong Discipline temporarily nulifies damage and creates a slow-motion effect whenever you're about to get damage during a dodge frame, allow you to counterattack without repercussions - makes fights against single powerful foes much easier as you can both nullify damage against your characters and deal massive damage against the enemy yourself. Meanwhile, Gizmo Supremo gives four special equipment slots that you can use to bring special gadgets (drones, healing aura, massive falling ball) into the fight - somewhat useful. Both of the Martial Methods are available to all characters, but only one can be equipped on any particular character. This new mechanic adds some diversity to combat, but it also makes battles far less challenging, to be honest.
The entire DLC is unvoiced and doesn't really have any decent plot or character building. The Labyrinth is basically a regurgitation of reused content with a little bit of new polish that gets old fast (and it takes like an hour or two to get from one save point to the next, unless you're skipping all combat and loot in favour of just speedrunning to the exit). It would be one thing if the DLC was a freebie or nearly-free, but it's definitely not worth the 10€ asking price unless you're specifically looking for some mind-numbing grind.