Blade of the Ruined King and Might of the Ruined King
I'm not sure if it's working as intended, but if you build a BotRK and Ornn uprgrades it into a MotRK, and you build another BotRK afterwards, the on-hit % current health damage stacks to an insane 16% current health damage on-hit. I wasn't recording, but I played against a kog'maw as ADC, and with him having both Ruined King items, and in the death recap, he did 300 damage with AA's + abilities, and did over 800 with items. I was playing twitch, and didn't build any health, and he only auto'd me 3 times. That much on-hit damage from items can only be explained by the supposedly "unique" passive on the items stacking because of it not being a named passive, and the items being technically two different items.
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the 8% stacks to make 16% damage