This guide will let you know where to get Reaping Spiral in Warframe.
Reaping Spiral can drop as a completion reward for completing certain bounties on Cetus/Plains of Eidolon.
Once you are on Cetus, you have to talk to Konzu and look for bounties that are level 10-30. Looking for the reaping spiral mod in the rotation reward pool, you can just wait until the rewards cycle if it isn’t in the reward pool. You can see when the next cycle will happen by looking at the timer at the top of the bounties menu.
It’s likely going to take you some time to get the mod.