Being a prey animal, a horse’s natural defense response is flight. However, there are occasions when standing their ground is a choice. Mares with more dominant natures will challenge any violation of their space comfort zone, especially if they have a young foal. New foals stick close to their mothers in the first days and weeks of their life. The protective instincts of the mares is greatly heightened at this time.
In the case of this mare, this is her lowest-threat-warning-face, based on that, I would not want to confront her dire threat response. The “dead eye” is an ominous indication of intent.