This is a requisition as opposed to a sharing of rules. While I realize that this will almost certainly brought up in new expansions, I can't help but feel it's appropriate to have them now.
There are Tzeentchian Sorcerers traveling on foot, Lords of Khorne walking into battle, and that just seems inherently wrong to me. Surely there must be someone who feels the same way?
I feel that I must throw some kind of suggestions out, so:
An extended Willpower test seems obvious, a -30 modifier for bending it to a purpose it wasn't suited for
A separate test based on the attribute of the particular god
Allignment being necessary, of course, a mark giving a positive modifier
And then the capabilities of the beast and its boons to you. Of course it has its own abilities, Screamers (Or discs, though that should be similar to creating a Daemon Weapon) being capable of flight, Juggernaughts and their rampant strength and violence, Beasts of Nurgle and their vile concoctions, etc.
How would one determine wether you are attacking mount or rider? Allocated by the attacker or the victim?
The Corruption, Infamy, and Experience requirements?
Their possiblitiy as Gifts of the Gods? Or is it too overpowered?
If it is listed, would the tests still apply? I believe they should, it would show the fickle nature of chaos, they gift you with the oppurtunity, if you're not strong enough to make use of it, then you deserve the punishment it dishes out.
Anyways, thoughts, input, all is appreciated