There are only two pieces of DLC for this game.
Triple Bonus Pack: As its name implies, this DLC contains three things: a new dream, called “An Old Soldier’s Legacy”, a ring that enhances attacks against machines and magical enemies, and an item called Memory Lamp that allows you to watch previously viewed cutscenes again on the Nautilus. There’s not much point to it, unless you’re a dream sequence completionist.
Seeker of the Deep: The second piece of DLC is far more substantial. It includes a new optional dungeon, the Experimental Staff Remains, with a superboss called Killalon at the end. It’s a giant crab-like robot operated by an adorable little kelolon scientist, Professor K, who believes that kelolons are the master race. You’d almost feel bad for harming it, but it packs quite a wallop, so ultimately, you’ll be quite happy to dispatch it. The rest of the enemies in the dungeon are no joke either, but there’s also new loot to compensate (including more Slot Seeds), as well as new Achievements. All in all, it’s a fun challenge, even if it doesn’t really offer anything story-wise.