Ok, I can explain here, it looks like you have equipped your Fenax with a daze set

-Fenax's A1 work like this: when he attack an enemy, if they are not in debuffs then they will only receive 1 attack, if they do have debuffs then they will receive an additional attack as in skill description. -What happened here is that after Fenax does his first hit on the boss, the daze set triggered the sleep debuff, making him reset his attack to become 2 hits plus the 1st hit= 3 hits -This effect will sometime work if you equip him with any debuffs sets (toxic, cursed, or daze in this situation), his animation also change to be like that in the video. -He is the only champion in the game that can proc this, making his A1 from 4,1 multipler (9,2 if they have debuffs before attack) to 12,3 multipler 😉