Necalli's overhead attack is best used in mixups. It can also be used as a 'wake-up call' against opponents who block excessively in a crouching position. It also functions as a Crush Counter. The Opening Dagger does decent damage and has excellent range.
'Opening Dagger' is a reference to the Tecpatl, a ceremonial knife used by Aztec priests in sacrificial rituals. Human sacrifice played a paramount role in the Aztec society to a point where it was believed the Sun requires human blood to rise everyday.