I wanted to make an aggressive legend that could play bigger cost units late, without devolving into Aurora slop. So I made Sejuani here to encourage swarming with medium and big units.
So for example your Sejuani can 'Ride' Anivia into battle, combining their might into a shared health pool you need to deal with to kill them both at once or not at all.
I also wanted to push her harder into having playable signatures and such so I made it uncounterable but slow and Body Sejuani can't be bounced to hand, while having a smaller base health pool.
I really want Sejuani to be kinda this disruptive force that while manageable, isn't just 'play defy' or 'play star-crossed' countered. She isn't spell immune, but you also aren't going to just toss her away, which I think is the biggest weakness of most bigger units like Volibear, that make them really kinda bad.
[This has been updated to be more clear about rulings of what is can be chosen and adds on defense to the Rider keyword, because it didn't make sense that she would dismount while defending.]
Thoughts?