This guide will find you a way to get the Rage in Warframe.
You can go first in the void; the Rage will drop from the containers/boxes in the void.
You can also go there since the grineer napalm’s that spawn in this mission has a chance of dropping the rage mod.
After capturing the target, you can find and complete the orokin challenge room and then leave.
The drop chance for Rage on both missions is pretty low, so it may take you a while to get it unless you’re lucky and get it quickly.