It's 1% of your Bonus Health. But yeah, there's been some... interesting phrasing pushed to the PBE recently, but hey, it's the test realm, it's not entirely gospel.
Doran's Shield used to block 8 damage (before resistances), and was only on autoattacks/single-target abilities.
Bone Plating would, let's say, on someone with Doran's Shield and the 65 Bonus HP from Resolve runes...
- Have a total of 145 Bonus HP
- Block 16.45 damage, pre-mitigation, for three hits at level 1.
So those two autoattacks and a Broken Wings from a Riven would lose 16.45 damage each, which is nuts early game. That's almost 50 damage just gone, poof.
It might not seem like much but boy it can add up bigtime over the course of a game, but it's especially impactful early game because nobody has any real base damage or damage scaling going.
... And this is assuming that it affects pre-mitigation damage instead of post-mitigation damage, because that would be a super strong rune!
I really wonder how that'd play with Kled, since Skaarl's health is treated as Bonus Health, and you get 1420 of that for just being level 18. Surprisingly tanky angry yordle man. Can't wait to mess around with it.