One of the ways to get to the new final boss (will be updated in the future)
 
 

Get to the moon!

So to get to the new area and final boss, you should kill mytrix (as usual). Then instead of charging up the ship, go to the START of the level. You will see a cute purple frog and a promt to “pet it”.
 

 
 
Pet it 10 times (1 lunar coin per use) and the portal to the final area will open
 

 
 
(also the portal can randomly appear on any stage. It is free, but you need to get lucky)
 
 
 

Is there any difference between this two methods?

Yes, there is! If you just find the portal after the stage 5, first you will be teleported to an area where you need to activate a couple of pillars before you fight the final boss. There is no such area in “frog ending”, but both will get you to the final boss and both will unlock the new character
 
 

 
 
Hope you enjoy the Guide about Risk of Rain 2 – New Final Boss Guide, if you think we should add extra information or forget something, please let us know via comment below, and we will do our best to fix or update as soon as possible!