First off: I am not a lawyer, and if you want a definitive answer you'd need to talk to one, preferably in software law.
Originally Posted by Ayantir
From a legal aspect, it's easy :
ALL the addons belongs to ZOS
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While the other portions are clearly Elder Scrolls IP, this portion isn't nearly as clear.
First off, the part under copyright is the API not the addons themselves:
As part of the ongoing Services provided to You, ZOS will make available an application programming interface (the "API") to allow You to create, download, enable, use, or associate Content, including user-generated Content ("UGC"), that modifies or otherwise provides enhanced features to the user interface ("Add-ons") for The Elder Scrolls® Online software-as-a-service product purchased by You (the "Game").
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The usage terms are related to using the API itself -- not addons, and you'll notice there aren't any provisions that state you concede copyright of any addons.
From there, it falls to regional laws for whether it's considered a derivative work or not, and that depends on the addon. For US guidelines, a quick google search turns up a paper (below) which contains a summary of several cases about user content in conjunction with games. Some are considered within the copyright of the original owner while others aren't, depending on the type of content. Throw in other regional laws and it's far from a simple cut-and-dry answer.
Personally, I'd suspect it'd be the API itself and use of any ESO assets, text, etc would be considered derivative, but any actual logic/algorithms wouldn't. It'd be possible to extract the ESO-specific parts of an addon to have a copy that is solely under the author's copyright.
http://www.academia.edu/1428208/Comp...t_and_Copyleft
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For the OP about displaying content on websites, it'd basically be whatever "fair use" covers at least in US. Which if you don't claim the work for your own and aren't selling it, it'd probably cover most of what you could do online. Something like Uesp might come the closest with putting up full text from books, quests, etc.