Casting Returning During Assassination

[ano]

afaik, returning is one of the spells that always goes off no matter what the opposition strength is.

You are mistaken. Shadow Seers are the golems' nightmare. As well as deep seers, btw.
But I'm still not sure this behavior is tied to hp. IIRC, Arch Theurgs have 8 hp normally, but golem can escape. Maybe it is some calculated value of "relative power" what matters when spending gems on returning.

[chrispedersen]

game calculates an "overkill" value, which you can see in the debug log. You could run tests to find out at which point gem use was no longer triggered.

[aaminoff]

Follow up question: what happens if you cast returning in an assassination battle in your own capital?