Before we start let's just get a few things out of the way.

First off, The Promethean is not an official Dungeons and Dragons product ... although you could be forgiven for thinking so, because the author did an excellent job of publishing it.  Second, The Promethean is not a full campaign supplement: it's just a class (with seven sub-classes and a multiclass feat), which lets you play a D&D character who resembles one of the Titan-hosting characters from Attack on Titan.

Finally, while D&D is the world's most popular role-playing game, and it is very flexible, it was designed for, well, dungeons and dragons ... not the steampunk fantasy of Attack on Titan.  As such it lacks rules for things like mobility gear, spinal fluid injections, or stats for the Titans themselves (although you can make your own using similar large humanoid monsters, such as giants or golems).  There's also no Paradise Island setting (with maps, NPCs, adventures, etc.)

However, if you're not looking to play in the world of AoT, and instead you just want your next D&D character to be more like Eren (or any other Titan-powered character from the story) the homebrew Attack on Titan-inspired class called "The Promethean", from author HOSTwritr, will give you exactly that.

The Promethean supplement includes eleven pages of content, not counting the title page and credits.  Many of those pages feature large (and gorgeous-looking) Attack on Titan art, but there's still more than enough detail provided to make this class a fully viable one ... assuming of course that you enjoy homebrew content.

The class starts out as a basic unarmed melee class, gaining the ability to do d6 unarmed damage (or more at later levels), and gaining a bonus to AC when unarmored. At 3rd level the class really comes into its own, with the "Transforming Light" power, which lets the character transform into a "much larger being" (ie. a Titan) if they are wounded.  The character can choose between seven possible titan forms for this power (each clearly inspired by the anime).

Future levels add more AoT-inspired powers, such as the level five "Collateral Damage" (which lets the character commit area-of-effect attacks using their massive size), or the level nine "Rejuvenating Fire" (which creates a burst of flame when the character transforms, similar to the explosions the Titans on the show create when transforming).