Free download of Squids Odyssey on Wii U if you've downloaded Squids Odyssey on a Nintendo 3DS that is linked to the same Nintendo Network ID that you have registered on Wii U.
Quote from the developer:
How Nintendo of Europe approach them for this promotion
''Indeed this is something new for Nintendo, they call it conditional sales. They [NoE] asked us if we'd be interested in this kind of operation and we said yes. I believe Squids Odyssey is one of the first titles or the first title to support this. Unfortunately, it's only 3DS -> WiiU for the moment, even though I know they would like to see this work the other way round (and we'd like it too!).''
Edit: Tried it out today and it works seamlessly. Once you purchase the game on your linked 3DS, when you go to the Squids Odyssey page on the Wii U eShop, the price will be shown as Free and you have the option to download. It takes up around 450MB