Legions of Chaos
Okay, remember back in Warhammer Fantasy 6e, when Chaos was under the name of the Hordes of Chaos? Warriors of Chaos, Daemons and Beastmen, all intended to be used together in one big horde? Well, with The End Times: Glottkin sourcebook, it's back at last, under the new name of Legions of Chaos.
Unlike Forge World's complicated Chaos Great Hosts approach, the Legions of Chaos are pretty simple. At their foundation, they're a plug 'n' play of characters and units from all of the other Chaos books; the biggest difference is explicitly spelling out that daemons can't get any leadership benefits from daemons who aren't of the same god, mortals & beastmen can't lead daemonic units, beastmen can get Chaos Marks again (2pts per model for units, 10pts for Heroes/Lords/Chariots/Monsters - except for the Mark of Slaanesh, which is 1pt/5pts depending), and beastmen get to roll on the Eye of the Gods too. Except beastmen can't become daemon princes anymore, so if you roll a 12, you - no, you don't get the Divine Greatness rule automatically, you get Damned By Chaos instead. Which, like the removal of daemon princes of Chaos Undivided is LAME!