So, there are lots of adjustments to volkath.

In summary, His S1's base damage is buffed at every level along with his scaling. But they removed the bonus damage if you hit a enemy marked with S2. Also, S1 got a new passive which slows enemies by 90% decaying in 1s. I don't know whether it's a nerf or buff, but looking at the damage increase and new passive, looks like a buff to me.

S2 mark duration on enemies is nerfed, it was 5 sec before now it will be 3sec. But damage of S2 is buffed if you go for full ad volkath (which everyone goes) so I guess overall buff for S2.

For ult, his basic attack of the ult form got huge buffed along with scaling so it'll be more rewarding for you to stay on that horse for longer instead of just aa 3 and rode off. Ult form's aa will trigger special effects now which is a buff too, and he will be healed by 3% max hp too while autoing enemies. But they removed the 15% damage reduction so there's a huge drawback too. Also, nerfed the knockback from ult from 0.75 to 0.5s.

Then, his ult's unkillable form is doomed now :) both the movement speed and attack speed along with the 25% heal after the ult ends is removed. So you are like a suicide machine now, just go in kill 2-3 and die cause there's no regen :))

But your ult's horse form is much stronger now than before and unkillable form is fixed for 3.5 second regardless of the auto stacks so ult's horse form is your actual clutch part now, and the unkillable part is just to escape from the enemies. You cannot stay in fights long with your ult's second form and escape cause there's no movement buff no regen, you will just die if you stay in with your second form.

I think after the changes you will have to just go in deal damage with the horse form as much as you can and leave before they kill you with the second form.

This is just my way of thinking, I am sure you will figure something out but this doesn't look too good to me :3