... what's a legacy?

I ask because I'm writing a document to help patch up plot holes in Heirs of Blood. Finale is... kind of a narrative mess that only kinda makes sense from a specific prior mission order.

Worst offense are these "legacies" that suddenly show up in encounter 2b, are not explained, were never mentioned prior, and have a weird esoteric function that requires a couple read throughs to understand the value.

So... what are these things and why do they do what they do? I've re-read the quest guide a handful of times and can't sift out any meaning to it.

Thanks in advance