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So Twin Fang receives reduced effects Rylai's and Liandry's. Additionally, Twin Fang receives no effect from Furor Enchantment, Muramana or Hextech Gunblade's Reload passive. This is intentional, as is stated in this thread.
I think the ability's tooltip should include this information. Because a search of the forums will show multiple threads mistakenly reporting these effects as bugs, and the reason for this is because this information is nowhere in the game at all. A player would only understand this by reading about it somewhere outside the game.
With Furor, Muramana and the reworked Gunblade giving no effect to Twin Fang, I think it's become important for the ability's interaction with items to be perfectly clear to anyone who plays Cassiopeia. As is, the skill is baiting players into buying items that are ineffective.