i think nyst a tower construct of some sort made by the anak for his purpose.
if you replay the section, after nyst say to have a event there is a dialogue box with xerces with only "..." the thing puzzle me the most is why empty his room? it isn't simple leaving all in it, an simulate a little use? this is a huge reg flag for something amiss for anyone with the ability to sneak in (and xerces know very well in the gathering there is at least one able to do so, we/simon group)...
Sierra in patreon comments say about skullpounder made the same arc of skullcrasher for some "nostalgic comeback" of sort. but the anak doesn't have done anything to control or guide skullpounder, he only give a little shard fragment, and not in a direct way, if i read the investigation dialogue right, anak influence begin only after the start of the gathering, with a bottle magically charged. in reverse nyst shard IS the anak shard subtely disguises.
i found more relevant is the ability to broke a fragment of the shard within him without killing himself (unlike estaven did), this mean the process can be mimic with the divine shard. perhaps yarra was right and tertia can be chiped off, or other goddess can give some fraction of their shards to other...