User blog comment:Some Argonian Named Kesch/Stormcloak Rebellion: Is Anyone Really Right?/@comment-24165787-20131113212045/@comment-13446185-20131115211341

Dovahsebrom I meant Tiber knew he could never conquer the Summerset without the Numidium.

It never says this

"The Underking tells him that, with the Tribunal dead, Septim might steal the Tribunal's power and use it against the High Elves (certainly the oldest enemies of Lorkhan, predating even the Tribunal). Summerset Isle is the farthest thing from Tiber Septim's mind. Even then, he was planning to send Zurin Arctus to the King of Alinor to make peace."

"This Numidium is what he needs to conquer the world. It is his destiny to have it."

- The Arcturian Heresy

Tiber Septim honestly didn't give a crap about invading Alinor. He was fine with just making peace with them. Then he got Numidium and knew he could just conquer everything anyways, so he did (except Blackmarsh).