Kalista on Howling Abyss

As playing Zac - Build consisted of warmogs, spirit visage, merc treads, giants belt, amp tomb Kalista has grieves, hurricane, bork, bloodthirster

Kalista managed to deal 51% in true damage and it was labeled under basic attack. Under it was also basic attack Physical for 24%.

Where is the true damage coming from? Bork is physical, Rend is physical, Sentinel is magic damage - and yes I was being hit by the bound soul as well. But there's a 10 second cool down on that % health damage (it was rammus)

And since I died within 12 seconds it could only happen twice. Max of 40% but I have magic resist.