Ahhhh! *facepalm* I should have searched the subtitles before posting this! There are several references (I think mostly by the Justiciar?) to Zhao Yunlan being the Lord of the Zhenhun Token/Guardian Token.

Pretty sure they don't explain what it is or what it does, though.

So... something intrinsic to Zhao Yunlan's position is a constraint on Shen Wei's powers? Huh.

(*swoons over your icon*)