[Spoiler] Let's discuss some interesting details of that "one of them" rift thingy.

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As you can see, you are connected to your body with a faint particle beam.

So far, u/Juniterio's theory is my favorite. Quote:

I think that the Thaumanova anomaly accident is basically how souls are made. Take 6 separate types of magic, merge it together and then condense it. Ley line magic is pure magic (all 6 types together) and it forms little sparks in bigger densities and large and sentient like living beings when in big densities - the anomalies. I'm waiting for Deroir to tie those things together with dragons (possibly recycling machines) and gods (the magic condensers?)

How do I turn into an anomaly?

I completed Burden of Choice, kept the stone (Priory FTW!) and had the class II rift stabilizer. u/nagennif/ thinks in his post that you need to switch your stabilizer to 9 7 for it, but that did nothing for me. I kept 9 7 though and just jumped up and down for a minute or something, and suddenly the damn rift procced the anomaly effect.

So, yeah. Fiddle around. Probably just a low RNG proc, may or may not be tied to the class of your stabilizer or some achievements/events.