Terminus is your sword.
It’s always equipped to you, so when you get to any aspect of the Red Judge (the big winged skull boss at the end of each level), do damage with any weapon of your choice, then when she only has a sliver of health remaining, either press
twice to draw your sword, or press
once for the same effect.
Then simply jump at or soar towards the Judge Aspect and get the final hit with your sword.