How should I react to my parents fight?

Today I stayed home with my little sister to take care of her because she was sick while the rest of my family went to church. My parents came home from church they started to fight soon after and it escalated quickly. I was in the kitchen making porridge for my sis but both of my parents began to scream at each other at the top of their lungs cursing in front of my two toddler little half brothers. My dad started to threaten my mom but i didn’t interfere because i was terrified and crying but i also trusted that my dad wouldn’t do anything stupid. I stood there like doing nothing and then my dad threw a metal spoon at my moms face really hard and it knocked off her bottom teeth and blood poured from her mouth. I was terrified and crying rivers and also confused because i didn’t know why they were fighting and my dad never acts this way, he is usually really nice and funny. I’m so confused because it’s way out of my dads character to act like this. My dad is actually my stepdad but he treated me and my siblings really kindly. I don’t know wat to do because I love my dad but i won’t and can’t forgive him for disrespecting/abusing my mom and knocking the teeth out of her mouth. They fight almost every month but it never gets out of hand like this. My moms last marriage was really hard for our family because we went through a rlly bad financial time and i don’t think we can handle it again especially now with two more kids. What should I/they do?