Multiply triangular. (And yes, a triad is another name for "chord.") The thing that Rogue and the Doctor trapped the Chuldur on in "Rogue"? Huge triangle. We may have seen Sutekh in satanic form.

Rogue soon revealed himself as a bounty hunter; he didn't explain who he was working for, but he did tell the Doctor that he would be paid handsomely for catching and killing the Chuldur ...