Fanart is the love child of two parties: The artist and the copyright owner of the original property or ideas the fanart depicts. Because I am declaring that the artwork I make is referencing, depicting, or relating to copyrighted properties that do not belong to me, then I am no longer the sole owner of that artwork. For example:

If I caption this picture below as [KINGDOM OF ASH SPOILERS] “Elide racing across a bloody battle field, desperate to save her love before a dam collapses and floods the entire field.” [END KINGDOM OF ASH SPOILERS] — you as the FAN of that work would recognize the scene, recognize those characters, recognize those EMOTIONS you felt while reading those scenes by looking at my interpretation, my fanart. You’re connecting with my art through the emotions that were earned by the Author, Sarah J. Maas, not me.

If I were to instead caption the picture as “A girl desperately riding across the field on a horse because she really has to pee.” I no longer am associating it with a copyright work, but it sounds a little less appealing in context, doesn’t it? Not the same emotional connection as before? Therefore, the only thing left sustaining its value is my artwork alone. ( Unless you really sympathize with the desperate need to seek a toilet. )

In the case of fanart, I would need the consent of both parties to sell my work for it to be within legal standings for Copyright Law.