Complex sentence: Natalie tries to attack Mia with a box cutter, but Mia takes it from her and uses the blade to split her own tongue in half, before planting a bloodied kiss on Natalie's mouth.
Split into simpler sentences: Natalie tries to attack Mia with a box cutter, but Mia takes it from her. She uses the blade to split her own tongue in half, before planting a bloody kiss on Natalie's mouth.