>>7534240Roboute Guilliman woke up, and revealed that before he was poisoned by Fulgrim's blade and put in stasis, he had ordered the Archmagos Belisaurius Cawl to create a new kind of Space Marine. Primaris Space Marines are bigger, stronger, and tougher than regular Space Marines, at the expense of being less versatile.
Guilliman starts a crusade to reconnect the Imperium that was sundered by the fall of Cadia and the ripping open of the Eye of Terror.
As he goes he forms new Chapters of all Primaris Marines, while also bolstering existing Chapters with them.
Regular Space Marines can elect to undergo the procedure to become Primaris, but it's very risky, 50/50 chance of dying
As for why some people are butthurt about it, it's because they're nerds and they don't like change
