"Oooh, so many riches for Starfinder! A scenario by Alex Speidel introducing new factions! A chance to meet with Miyu agaiin in a higher-level scenario -- though not high enough for me, alas. And Road to Reconciliation sounds interesting too."
My best guesses are:
Rasheen -- honestly, she was a treasure hunter who brought back tons of artifacts. It's very possible that she acquired one under less than honorable circumstances.
Historia-Prime -- He's stolen other things, though I can only see him as the villain here if the artifact did something cool that let him maintain his deceptions or dominate his proteges.
First Seeker Lezosk -- I mean, the dude built the Perplexity. That ysoki was warped.
Who do you think was responsible?