If you downloaded a brush that doesn't look like Marc's (e.g., it is a weird square or doesn't have opacity changes), you likely downloaded a fake file. Delete it immediately.
Marc Brunet Advanced Brushes are a set of high-quality, custom-made brushes designed for digital painting and illustration. These brushes are created with the goal of mimicking traditional media, such as watercolor, oil, and pastels, while also providing a range of unique, digital-specific effects. The brushes are highly customizable, allowing artists to adapt them to their specific needs and style. marc brunet advanced brushes free free
The pack usually covers sketching, shading, and special effects. Highly recommend grabbing it while you can! If you downloaded a brush that doesn't look like Marc's (e