A small amount of sediment has formed and the vinegar has darkened.
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Nancy is a trusted home cook.
in theory, as the French say, some food-safety or duration sites advise 4 months max.
in practice, as they also say, I've used prepared horseradish (similar to yours, in vinegar, but grated instead of whole) between 6-12 months and found nothing wrong with it except a little dulling of the nuclear-strength hit you get from freshly made or freshly opened.
I would open the jar, check the horseradish for signs of mould or other degradation and - if no such signs - taste and use.