Early on while participating in this season's Winter Festival, I noticed that I would occasionally fish up "treasures" during the fishing contest. As treasures are usually what Neverwinter calls vendor trash, I didn't think too much of it.
They weren't marked as vendor trash however, so I eventually looked them up and learned that there is a vendor who trades these for various campaign currencies. He's located on a pedestal behind the daily quest giver in Twilight Tor and has a somewhat unusual-looking face, which immediately made me wonder whether he was modelled after a real person... But I still shrugged it all off - I'm generally fine just playing through the campaigns the normal way.
Then, two days before the end of the event, I learned that ten fished-up Treasures of Tiamat can be traded for one Linu's Favor, the annoying-to-come-by campaign currency that has stopped me from officially completing the Tyranny of Dragons campaign to this day. And this option has been in game since 2016???
I went on an emergency fishing spree on my main, but even with the top fishing pole it took forever to get enough of the right treasures, so I eventually gave up on it again. I briefly considered buying a few sabiki rigs from the Zen store (special lures that increase your chance of getting a good catch), but then told myself that this wasn't much less "paying to win" than just buying full campaign completion.
Maybe I'll remember to start fishing on my main in time next winter...