The damage is not an aura or a debuff, therefore it can be beneficial for multiple champions on the same team to build the item. However, the passive is unique meaning building it twice on the same champion is pointless.
The damage is magic, so it benefits from magic penetration and MR reduction (it combos well with Abyssal Scepter).
The damage does not proc the Liandry's Torment passive, but it will proc the Morellonomicon passive.
The damage it deals does not benefit from spell vamp (and it wouldn't benefit from Life Steal because it's not an auto-attack).
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awesome! exactly the secondary questions i was going to ask, too