Nostalrius Begins - Quality wow vanilla realm (1.12)

hello this has really been bothering me and id really like to figure it out what addon is it and how do i get it to happen in my game where it announces on screen when you crit with a spell by showing "spell critical" heres a example of what im talking about if you dont know

https://www.youtube.com/watch?v=7FFcgGWxk_k

in that youtube video of a rogue meandro whenever he crits it would show spell critical thats what im trying to figure out to happen in my game when i crit please help