where did you hear this?
Only thing that I can come up with is that now with the new Generals HandBook 2017 you can use 20% of your list value as allies, so then you might be able to use beastmen with Skaven (at the moment I don't know if beastmen are able to ally with Skaven or viceversa.. every faction has it's list of possible allies)
I think i may have been confused because of this:
If this is because of my skaven/ beastmen army combo, it's only that I can. I didn't realize that only 20% can be allies but that's what I will aim for. It's just some fluff from the Warbeast book from the realmgate Wars. Nothing official
On the flip side, it kinda makes sense that furry's would team up...
Wasn't sure if it was a new warscroll (is that what they are called) with some fun fluff that would have taked about what and how.
I always found the demon and treespirit army from TET interesting, and fun theme combinations are what I like to see...
I don't know how much you know about Age of Sigmar, so I will explain as if to a new player
The default core rules of Age of Sigmar, let you pick whatever units you want wiht no limits. It's up to the players to decide on any limitations or restrictions, or battleplans(scenarios). As of the General's Handbook 2016, that is now what is called Open play. There are 3 styles of play, Open play, which is do whatever you want. Narrative, where you are playing within a story style, so you're representing the battles of the fiction, the published campaigns, Path to Glory campaigns, and so on, there are still no restrictions, unless you choose to, though there sort of are with Path to Glory.
And then the 3rd style is Matched play, which is intended for the competitive tournament style play, which has restrictions, and points.
In matched play, you must pick an allegiance, which can either be a very specific allegiance, such as Brayherds (The man sized beastmen stuff), or you can play with a Grand alliance allegiance. So in a Grand Alliance Chaos allegiance, you can take any units that have the CHAOS keyword, so in this case you could take some Skaven and Brayherd units together with Daemons of Nurgle, or Khorne Bloodbound.
Picking an allegiance gives you access to allegiance abilities, of which there is a battle trait (a sort of army wide rule), command trait (an addition rule for your general), and artefacts (for all intents and purposes, they are magic items).
There are also some smaller faction allegiances you can choose (In matched play all of an army must be chosen from the same allegiance) and Brayherds are one set that has it's own allegiance abilities - command traits etc, that are more suited to that faction, and therefore you get slightly better benefits. You can still allies of up to 20% of the points, but only specific other factions, and as of Generals's Handbook 2017, none of the Skaven factions can be an ally of Brayherds or vice versa).
Many people still choose to use a lot of the matched play rules to build armies even if they then play Narrative games or Open play games, as the GHB says that basically all of the GHB content is optional.
