when will tenshin sell the next umbra forma?

[iJinya]

^ - ty for the answer <3

[Finni]

Probably next week.
https://warframe.fandom.com/wiki/The_Steel_Path#Steel_Essence
Scroll down to "Rotating offerings every week"
You'll see that right now the Shotgun Riven is the active offering, before that it was the Rifle Riven, so it should start again at the Umbra Forma Blueprint next week. That's my guess I don't have the rotation in mind, but the Wiki should be correct on this.