Going to list the problems with this item that I see after looking at it
- Too much AD for how strong the item is defensively, should only give around 35-40
- Item is basically Bone Plating but way better. ( Bone plating taken on anyone who can go Resolve secondary because of how OP flat damage reduction is pre-mitigation)
- with 150 bonus AD ( that's average Bonus AD I'll see during a game on either class relative to when games end ) the item gives 32.5 damage reduction, that's already as good as boneplating and it's on perma. It can also be combined with bone plating to further the effectiveness. In a teamfight 32.5 x 200% = 97.5. **broken**.
- Lategame it's going to make Assassins hard to burst in teamfights which will lower the skillfloor of playing assassins. (Pissing off lower elo's)
- bruisers going to burst adc's while soaking tank level's of damage.
What I suggest is to make the passive only work against auto attacks, that way it will be be doing it's job while not screwing over casters since flat damage reduction is less effective against dps while better against burst. (obviously because dps will continue to deal damage and net overall more damage dealt than burst)