I like this item, but its weird for it to deal damage since it will make players want to snipe players off rather than use it for the heal as a support item. I am thinking rather than damage maybe more power in the heal aspect and change up the stats a bit.
For example Ability Power instead of Health, need more AP support items and support tanks are getting a brand new defense item for themselves.
The heal could be something like 10% max health plus 130 +20 per level of targets +50%AP and only heals allies removing the damage entirely. Now creating a % max health true damage item makes sense since supports want to attack the tank with their backline, but needs to be implemented on a different item entirely since items need to have a focused identity. They become one dimensional this way, sure, but items are dynamic in the purchase of deciding what item to buy rather than the item itself - so it be better to make another support item that deals 10% max health as true damage in a burn form than implement it in a heal active.