Does Rikimaru's passive Backstab stack with critical damage?
Is there a chance that my damage will be equal to this
Normal attack + backstab + critical damage
The short answer is yes.
Here is the description to the Backstab skill from Rikimaru
That said, all attacks from behind will do extra damage that can be calculated (ignoring armor resistance) by adding the ammount added to the base damage from your character.
Here is some information about how Critical Strikes work:
Backstab and critical strike can proc in the same attack. Period. What's not clear here is when will the critical strike bonus be calculated.
The backstab skill adds bonus damage to your atacks based on your agility, and critical strikes have a chance of x to multiply by y your atack damage.
Lets say critical strike damage is the last parameter to be calculated. That would lead us to something like this ( I picked the values )
On the other hand, if Backstab bonus is added after the Critical Strike sums up, we would end up having :
Whether the Critical Strike is calculated after or before Backstab sums up, is not clear, but they CAN proc in the same attack.
Based on my experience from Dota 1, the backstab does not proc at the same time as the crit damage.
This is how it roughly goes :
1- You hit the ennemy from the back