This here was the meat of the gameplay for the dlcs and it is also my personal favorite of the bunch. There is slightly more story here than the last but it's still not the focus, with the gameplay, spending most of its time on a roguelite mode, being its focus.
The meat of this dlc takes place within the Ancient Chaos Shrine, where Jack and his friends are spending time after the base game, and previous dlc, end. The main hall has been converted into a hub area of sorts where you can heal up, buy items, craft new equipment and talk to NPCs in-between missions to prepare for the next one. Also yes, you heard that right, you can actually buy items this time. Unlike the base game and first dlc where potions are refilled automatically when you rest at checkpoints, this time around, you need to buy them with gil, for the most part. And not just potions either, we have hi-potions, Ethers, Elixers and more available for purchase. Although, admittedly, the selection of items is still limited. More NPCs will arrive at the shrine as you progress, and completing the quests these npcs give you, as well as giving them items, will not only grant you passive buffs, regardless of your job, but also, assuming their storyline is complete, will grant you the option to summon them in battle with a monster bell accessory.
Now, to progress the dlc, you would need to enter portals to other dimensions that are appearing within the shrine, and complete the task asked of you. Different portals have different tasks and different levels of difficulty, and while within these variations of the existing levels, you will also get "chaos events", and "chaos monsters", which can range from beneficial to you, to horrible for you (Imagine trying to catch a fleeing cactuar, on a timer, and a bunch of tonberries randomly spawn in front of you). The stages are ultimately still just repeats of existing stages, but now that you have mission modifiers, an element of randomness and the risk of losing everything upon failure, it stops being grindy and turns into more of a true postgame experience.
As for job changes, we get the Blue Mage job, which might seem weird, given that you could always steal abilities from enemies, but trust me, blue mage makes it work, and we get Summon Affinities, to add to the existing build variety. These basically just say that the equipment takes of the affinity of a summon from a previous final fantasy game, and the higher the affinity with it, the better the effect. So having a high affinity with Phoenix will allow you to charge Phoenix's ability faster. Phoenix's ability being a free revive whenever you die.

As stated, this is a roguelite, so to move on to the next floor, you need to earn a certain amount of points. More difficult missions grant more points, and failing a level will lower your points. Progressing to the next floor will refill your health and allow you to progress further, whereas having your score hit 0 will cause you to "fail" the floor, and reset your progress. All of your items, npcs, sidequest progress, passive buffs etc. will be lost upon failure of a floor. From here, i'm pretty sure you could technically choose to start from the floor you left off on, but I would recommend going back to floor 1, if you haven't made it to far in.

You don't need to reach the end of the labyrinth to complete the dlc, there are plenty of floors behind that point, and there are some added missions to the normal level select screen as well. Overall a very enjoyable experience for anyone wanting to further push their time with this game, and doubly so if they like roguelites.