It took the numidium to securely win the unified empire the first time. The southern provinces and many neutral royal houses were not fully aligned with the Septim dynasties, (the Aldmer being openly hostile to human rule, and very very angry about the Alyssian Order and the Marukhati Selectives giving their deities literal schizophrenia) so Tiber Septim, having beaten down the Tribunal, and being gifted the Numidium from them in exchange for the armistice permitting them semi-autonomy, used said giant stompy robot to boot stomp the shit out of everyone and everything that stood in his way.
It being a mythic construct INTENDED to change the way the mundus works (The dwemer intended to use it to remake the world the way THEY wanted, but failed miserably, destroying their entire species except one), its invocation is MUCH MUCH MORE than just a giant battle mecha stomping on everyone. It also literally re-writes history, and restructures reality when it is used. THAT is how Tiber Septim became the mantled persona of the missing Shor/Lorkhan/Seth/Shezzar/other divine slot.
The robot is presumed destroyed, but is really at the bottom of Illiac Bay, if I am reading the various outcomes of Daggerfall right. An army of imperial aligned wizards could theoretically work to recover and restore the construct, and BOOM-- It's ON like Donkey Kong all over again.
The Numidium is one of the 8 "Tower copies" created in mythic representation of the act of the mundus' creation, which is what gives them their myth-altering powers. It is also why the Aldmeri Dominion is INSISTENT on controlling the imperial city-- White-Gold is ANOTHER of these tower-copies, as is High Hrothgar, the Crystal Tower in their own dominion, The walking tree in Valenwood, etc. If you note, the aldmeri dominion is VERY aggressive in trying to gain control of these places. If they can control them all, they can ensure that what the Marukhati did never happens again, and they are poised to complete their plans to reforge the world as they see fit.
(For further reading, see the 36 lessons of vivec, the books on the alessian order, and commentary from Micheal Kirkbride; one of the lead authors of the lore of this universe.)
When Martin Septim died, and the mythic powers associated with the septim bloodline failed as a consequence, ONLY THEN were the altmer in the south able to begin their bullshit. Which they did, IMMEDIATELY.
But that's enough derail in the FO4 thread.