Who are Space Marine 2’s new Chaos antagonists, the Thousand Sons?

polygon.com:

When Space Marine 2 was first announced, its debut trailer led players to believe the biggest enemy to fight would be the hungry hordes of Tyranids swarming the Imperium. They’re still a major threat, but the latest gameplay trailer revealed the game’s other antagonist: the Thousand Sons, servants of the God of Change. But who are these bad guys, exactly, and why are they such a threat to Titus and the Imperium of Man?

Over 10,000 years ago, before the current grimdark setting of Warhammer 40,000, the Emperor of Man created 20 Space Marine legions. Things immediately started going off the rails; two legions were purged from history, their fates unknown. A civil war called the Horus Heresy split the remaining 18 legions 50/50, with half on the side of humanity and the Emperor. The other half, including the Thousand Sons, ended up siding with the extra-dimensional and wholly malevolent Chaos Gods.

The Thousand Sons are arguably the most tragic traitor legion, and some fans have argued that their primarch, Magnus, did nothing wrong. (He absolutely did a whole lot of things wrong, but that’s a whole other article.) Each legion, loyalist or traitorous, has their own specialization; the Imperial Fists love to fortify, the Raven Guard is the best at stealth, and members of the Death Guard are tough to the point of near-invulnerability. Magnus and the Thousand Sons are top-tier pyskers; these guys aren’t just Space Marines, they also have a ton of space wizards in their ranks.

Back in the days around the Horus Heresy, Magnus the Red — who enjoyed a lot of freedom and power as one of the Emperor’s sons — spent a lot of time mucking about in the Warp. What he didn’t know, because his dad never clued him in, is that the Warp is the