Space Marines. They're pretty much the most iconic thing in Warhammer 40k. They've also got a new codex that you can order from Games Workshop. There's also a supplemental codex for the Ultramarines and White Scars. So, if you don't want Space Romans and instead want Space Mongols, you can pick that up. There's also a few new minis and some chapter dice if you prefer.
From the website:
The Space Marines are numbered amongst the most elite warriors in the Imperium. Created by the Emperor himself from the genetic material of the demigod Primarchs, they are the ultimate soldiers for a cruel and terrible age. Inside this 192-page hardback codex, you'll find everything you need to unleash the Space Marines in battle, plus thrilling lore and art to inspire your own collection.
Inside this book, you'll find:
- The history of the Space Marines, with a look at how these genetically engineered warriors annihilate the enemies of the Emperor with terrible force
- A Space Marines bestiary with background details for every unit and Character featured
- A showcase of stunningly painted Citadel miniatures
- Rules for Space Marines units that are available to every Chapter
- Stratagems, Warlord Traits, Litanies of Battle, Psychic Disciplines and Chapter Relics that define the ways that the Space Marines wage war
- And more!