item let's you sustain as though you dealt true damage (active)

Active for next 5 seconds the amount you lifesteal/spellvamp will heal off the supposed amount of damage you would deal if the damage you dealt were to ignore all of the targets's resistances.

A bit like Warwick's passive:

_The healing portion is independent from the damage portion of the ability, meaning the heal is always the same regardless of the damage dealt to the target. _

Doesnt have to be all the damage... Theres any number of variations that could be applied to a lesser degree. Example instead of active... it could be passive but instead of being all of the damage treated as if it was true damage, instead it ignores 30-50% of enemy resistance.

Could be on any number of item be it offensive or defensive. would be more for regen champs like Mundo or singe. Spirit visage when all you have is say bortk on jax is underwhelming.

How about as an active on ?

I think this could help restore order among the bruisers and tanks.

I think it makes sense on bruisers as they don't tend to itemize a lot of penetration items. The item would not stack with any armor pen or % armorpen. So not so much for ADCs. Especially if its on Zeke's herald.

Or maybe it could, on second thought I don't see why not