u/Breakoutofyourshell

>The planet they passed was never a destination, since its star had already entered its twilight.
Its biosphere and atmosphere had been utterly destroyed. The surface held nothing but cooled masses of molten rock.
A supernova had caused the devastation.

—As Heaven and Earth Are Made Anew

Nibelung, once the sun of Teyvat, went supernova and became a black sun—a black hole. The black hole depicted in the backgrounds of Dainsleif and Surtalogi is the remains of Nibelung.

Evidence 1: Ivanovna’s code is "J" instead of "I" to avoid overlapping with the other members, so the last member should be someone whose name begins with "I".

Evidence 2: For some reason, a giant clock appears in the background of the Hexenzirkel trailer.

For these reasons, the last member of the Hexenzirkel should be Istaroth. Q.E.D.