He's been in the quest before, actually.
He also featured heavily in the Battle of the Bone Gate. I'm pretty sure I gave his name somewhere. Can't find it ATM, as I am just on my phone right now. But he's really, really big. In the Battle of the Bone Gate he got a big spear shoved into him and punctured by what seemed like 100 arrows, and largely just went 'ow' and then kept fighting.
For the record, he's the largest Great Stag in the Empire. There are others, like the Taal-blessed one that acknowledged Magnus the Pious leading up to the GWAC, but this one is bigger.
Specifically, look at this moose:
View: https://youtu.be/HNfetnUwOUo
Naraiel's Great Stag is about half again as tall, and four times as muscled/thick with sheer fur and mass. When I said she can ride stop him by just standing outright, I meant it was almost a mobile platform rather than a comparatively skinny warhorses back.
And, you know. Being shaped like a Great Stag instead of a moose. Someone posted a Total War Warhammer pic which covers his general shape and what not, only you gotta scale it up as mentioned.