"Fighting witches is not all there is to it," she says. "Our being made into magical girls in the first place delays the death of the universe itself. We are an energy source. He never explains that unless you ask." Homura pauses. "But I suppose that does not matter now that our universe is gone."

She pauses. Mami just had to ask, didn't she? Homura looks uncomfortable for a brief moment before her composure is regained. She radiates calm, radiates a rock heart. She speaks, and her face is so perfectly blank. She's scared of what Mami might do, but if she refuses to say-- who knows what will happen.

"We, magical girls, are immature witches."

There's no other way to put it. Any other way will boil down to the same thing, so Homura puts it bluntly. Her voice is so very small, so very quiet, and she carefully ignores Sayaka's question as to how she knows what's going on.

"... When our soul gems dim and become completely black, we become witches."