Rakdos Charm + Torbran, Thane of Red Fell

Hey guys,

I have a question about [[Rakdos Charm]] and [[Torbran, Thane of Red Fell]] interaction.

For example, if my opponent has 5 black creatures, and I play Rados Charm for the third option (the one that deals 1 damage per creature), is my opponent taking 5 damage, or 15 damage?

I'm confused about, if the source of the damage is the creature (black source) or rakdos charm (red source) in this case.

Thanks in advance!