Darkening of Tristram has just started. Can I know the butcher pet can only get if i 1) use a lvl 1 character and kill Diablo or 2) i start the event with a lvl 1 and work up lvl til i kill Diablo?

if option 1, does that mean i cannot lvl up thru the event?

I know noob question but I havent start the event yet so like to make sure before i start. Thanks for any help.