grimly determined expression

The old man's face still had that grimly determined expression, and his pace had not slowed.

The moonlight showed his grimly determined expression.

She also wore a grimly determined expression . . . along with her swords, garrotte, and bandolier of throwing stars.

The murderous glare melted off his face, and what replaced it was not the flat, grimly determined expression with which he had entered the house, but the glazed and slightly quizzical look that signified activation.