Why does my cat start screaming and then have an extremely aggressive outburst right after?

background info:
we adopted him a month ago.
he is 8 years old
only one previous owner and i believe there was no abuse or anything, he’s very loving and affectionate

problem: he likes to look out the window and every once in a while he will begin to scream. it sounds like a child being murdered and its chilling and very frightening. it’s happened a few times and we always thought it was because he saw another cat.

today, it happened while i was in the kitchen and he was there with me looking out the window. he started screaming and i looked outside, there was nothing that i could see and then he pounced off the window sill and came at me and attacked my leg ferociously for a few seconds. he made me bleed! somehow, i’m not sure, because he has no teeth or front claws. and then he flew into the other room and started attacking the blinds, or thats what it sounded like anyhow. i was too scared to go see!!!!

anyhow, have you ever experienced this, cat owners? i’ve had many cats throughout my life (family cats, not my own personally) and i have never seen anything like this or heard of anything like it! google is doing me no good at all for the first time ever. help!?!?!?!?!!