Important things we've seen from mordekaisers new ult

it has been used to dodge brand and swain W so any delayed spell cast can be avoided(cho Q, Viktor stun, singed W etc) This functions even if you target the champion who casted them.

IIllaoi's passive tentacles do not carry over, possibly her ultimate tentacles as well but in the video she was ulted mid cast of her ult. Presumably however they wouldnt come with her if she casted it before mordkaiser did. This should naturally translate to other champions leaving garbage on the rift like zed with his shadow clones and heimerdinger with his turrets.

The cast range is very small but not actually melee, it appears approximate or slightly smaller than Aatrox's first Q

You cant actually walk out of the circle

You cannot see mordekaiser, or his target, from the rift until a few seconds before the ultimate ends and only then as red particles rather than the champion/s

The stat steal only works for the targets primary stat rather than all of them, in other words you wont steal the AD from an AP champion even though they have base AD values. Whether you steal health or resistances as well im unsure.

Likely some more things that i've missed. Not only is his ultimate supremely powerful when used correctly but the outplay potential is insane against certain champions.