The issue seems to happen because you have an n-gonal surface (the side of the sword has more than 4 edges). Usually, when renderers encounter faces that have more than three or four edges, they get confused.
To fix this, try to select the face and 'triangulate' (ControlT) it. This will split the faces into triangles, making the shape less ambiguous for the renderer.
If this doesn't work, you'll have to manually cut it up with the
knife tool (K) until only quads and triangles remain. If you do this, try to make a uniform pattern (don't just cut randomly).