I Transmigrated into Warhammer, and I'm Batman?

As the thumping electronic music played, I was serving the Emperor when the machine spirit suddenly went berserk. And then, just like that, I was drafted! A Primarch's son, the Great Crusade, and a multiverse series. I've only played Darktide and the Space Marine games, and watched video creators introduce the setting, the ones about the lobsters and such. I'm only vaguely familiar with Warhammer, so if I get any of the lore wrong, feel free to point it out and I'll correct it as soon as I can.