i think he has only 2 rage forms.
1st is activated when his life gets lowered fast and in 1st form he gets some bonus atk speed,mby higher chances for bash and crit and does partialy pure dmg,
2nd is activated after a long battle time and he gets high atk speed ,speed, his dmg is totaly pure(when i tried to kill him with reflect he was hiting me but he was only regening life he didnt get return dmg at all and i had 2 impale armors) also he must be getting very high chances for bash and crit coz he didnt let me move at all i was all the time bushed and he did way too many crits
that is death aura it does dmg overtime and slow and get it when you are close to him, if you dont have high regen or good life steal it does plenty of dmg for exaple if you have a supporter with no defence-regen items you can prolly die just from the aura (also mby it become stronger in the rage modes not sure about it )
if he hits you, you seem kind of taking sth. like poison dmg over time very fast and you are dead after seconds, even if he doesnt hit you