So, very potted version of The Story So Far.

Sigmar has kicked off his Realmgate War.

Having caught Chaos forces entirely by surprise, rapid gains were made, and each seized territory reinforce by auxiliary forces from Azyr (millions upon millions dwell there).

Whilst Archaon got his Ming On, and set about driving back the Stormcast's early gain, he's also plotting to finally invade Azyr itself.

In the meantime, Sigmar has been planning to besiege the Eightpoints/Allpoints, which is presently Archaon's stronghold - the point connecting all the Mortal Realms. Whoever holds it has a massive advantage, as they can send forces wherever they're needed. And he's been seeking to reunite his pantheon, starting with Nagash, Alarielle and Gorkamorka.