Warseer is one of the foremost forums related to Games Workshop. It is particularly known as a hotspot for rumormongers, although it also has active fluff and crunch sections for both Warhammer 40,000 and Warhammer: Age of Sigmar.
Warseer is also known for its extremely strict rules, to the point that the mods are frequently compared to Commissars (the mods refer to themselves as the Warseer Inquisition). They WILL delete and edit your posts, and they are known for locking threads whenever they stray from its stated topic (even if a single poster or two gets off topic). They're so stringent about copyright that linking to a site that links to another site with leaked images from White Dwarf is not allowed. They also aren't fans of naughty language, so you can't link to 1d4chan or /tg/.
It should be noted that Black Library editor Laurie Goulding has gone on record stating "I hate Warseer and all who post there" (actually, it was even harsher than that, but we can't have bad language, now, can we?).
/tg/, by and large, doesn't know about Warseer, and most of Warseer doesn't know about /tg/; for instance, using the term Failbaddon will be criticized intensely. Still, it's a good site for rumors (though not as good as Faeit 212).