Learn English – what’s the antonym to “windfall”

Depends what you are looking for. If you were meaning the type of rule the king had, the antonym would be something having to do with democracy, like:

egalitarianism

However, if you are asking about the harshness of the king's rule, then you would probably use:

clemency

Seeing just the title of your question ("What's the antonym for Schadenfreude?"), my answer would have been "Mitgefühl" (to keep it in German) or "compassion" (English), since I'd say that Schadenfreude is the absence of compassion.

Having now seen your description as wanting a word to express "joy or pleasure derived from the happiness of others", I'd say "Mitfreude" would be it in German, and "sympathetic joy" would be the closest I can think of in English (couldn't find a single word, though a bit of googling did turn up "Mudita" as per cornbread ninja's response, so +1 from me).

Edit:

Mitgefühl means "sadness derived from the sadness of others".

Schadenfreude means "joy derived from the misfortune of others".

Mudita/Mitfreude means "joy derived from the joy of others".