* Alliteration: The repetition of the "s" sound in "softly" and "stole" creates a gentle, almost secretive sound effect.
* Assonance: The repetition of the short "o" sound in "softly," "stole," "treading," and "toes" creates a feeling of hushed movement.
* Imagery: The phrase creates a vivid picture in the reader's mind of quiet, stealthy movement. The emphasis on "toes" makes it clear that the characters are trying to be as quiet as possible.
These devices work together to create a sensory experience for the reader, allowing them to imagine the scene and feel the quietness of the characters' movement.
It's also possible the phrase is using personification by giving the act of "stealing" to the characters, implying they are taking something quietly and perhaps illicitly.
Without context, it's difficult to definitively say which literary terms are most prominent. But all of the above contribute to the evocative nature of the phrase.