[Well if he's asking what it is, it's unlikely he is a baneling, so she explains - but quickly.

The growling doesn't seem to shake her up, but her eyes narrow a bit.] Banelings, servants of the Keeper of the Underworld. [And to clarify:] In order to remain in the world of the living, they have to kill for the Keeper each day to maintain their lives.

[She adds, carefully:] They exhibit...symptoms similar to your own.