You've had subtle (and not-so-subtle) hints and clues about the Totem, Numidium and Mantella for some time now. To review, Tiber Septim conquered Tamriel using something called the Numidium, which was created by his Battlemage. The Numidium was powered by the Mantella and was controlled by the Totem. Lord Woodborne had the Totem, Queen Aubk-i knew he had it, and it later disappeared. The Underking wants it, the Emperor wants it, Eadwyre wants it, Gortwog wants it, Akorithi wants it, Gothryd wants it and the King of Worms wants it. And you have it.
You just snatched the Magical Dingus of Ultimate Power from Daggerfall, so everyone is going to be looking to you to give it to them. You have a year and a day to decide who gets it.
One of the great things about Daggerfall is that it does not have a static ending. You get to choose how the quest comes out. After you've collected the Totem, it will speak to you. It says it will only work for certain people. You're not one of them. Go ahead, try to use it and see what happens (save the game first). Your job is to decide who gets the Totem.
Nulfaga (after you have already delivered the Totem) will quote a riddle which details the recipients. It would have been better if you could have encountered the riddle beforehand, but this is the only place that it comes up. You have seven choices according to the riddle:
Here's my own interpretation of its meaning:
"First is one who killed, but did not kill another": my best guess is that this refers to the King of Worms. I get it only through process of elimination, but it makes sense in a round-about way with respect to the Lich's Soul quest. By his order the lich was killed, but the soul was trapped, so it wasn't really killed. The King of Worms promises you fame if you will give him the Totem and the dialog agrees with that, but nothing else seems to happen. The King of Worms will use the power of the Totem to become a god. The strange part is that after completing the Mantellan Crux, he's still under Scourg Barrow and there are no new temples to join.
"Second is one who is two devoured by its young": Who else but Eadwyre, king of Wayrest, who heads the most dysfunctional family in the Bay. Eadwyare will promise you gold (if you're on his good side), but won't deliver. In fact, he'll sic the guards on you (not much of a fight, actually, so don't be expecting any excitement at this point). Eadwyre will use the Totem to break free of the Empire.
"Third is one who made one slave and many free": My best guess for this line would be the Underking who made Numidium and helped establish the Septims as emperors of Tamriel. The Underking will give you the Necromancer's Amulet if you give him the Totem. Unlike the rest of the people who want the Totem, the only power the Underking wants is the power to die.
"Fourth and Fifth are two who compete for mud": The rulers of Daggerfall and Sentinel, Gothryd and Akorithi. Daggerfall and Sentinel were fighting over Betony at the time that Lysandus was killed. Each will use the Totem to overthrow the Empire, but they will give you 100,000 GP if you give them the Totem. There is a persistent bug that prevents Gothryd from sending you a letter, but be aware that you can give the totem to him if you wish.
"Sixth is one who brings a home to the homeless": Gortwog, the orc leader who wants an orcish homeland. Gortwog is supposed to give you a random artifact for the Totem and the dialog comports with that, but nothing appears in your inventory. I suspect that this is a bug which has never been fixed. Gortwog will use the Totem to create the free state of Orsinium.
"Seventh is one who lords all, but does not lord at all": my best guess is that this refers to the Emperor, titular head of a vast empire ("who lords all"), but whose writ runs mighty thin outside of the Imperial Province ("but who does not lord at all"). The Emperor will give you the Warlock's Ring if you give him the Totem. He will use the Totem to bring the rebellious provinces back in line and reestablish the power of the Septim dynasty.
Now comes fun time. Over the next few weeks you'll receive letters from the people who want it. Well, you'll receive letters from people where you don't have negative rep. If you've got negative rep with any of them, they'll send assassins to just take it away from you.
I can already hear the "ka-ching" of the cash register going off in some of your heads and see the dollar signs in your eyes, so let me satisfy your avaricious curiosity. Yes, the assassins work pretty much like they do in other quests: each group has the same stuff as the group before it, plus a few items. The truly greedy will capitalize on this by knowing who will get the Totem in advance and then making sure that they have negative rep with everyone else. This will produce a flood of assassins on a daily basis, and a ton of goodies for you to sell off. A word of caution, though. Some of the assassins will be genuine Assassins (poisoned weapons and all), so be able to cure poison. The rest will be normal baddies (Knights, Archers and the like), except for the Orcs that Gortwog sends.
Remember, you have a year and a day in which to decide who gets the Totem. If you need to build up levels or work on skills, you've got plenty of time to do it. For once you've got more time than you'll ever need to complete a quest. Use the time wisely and the rest of the Main Quest will be a snap.
Once you decide to whom you want to give the Totem, take it to them. As soon as it's delivered you'll get a telepathic message from Nulfaga telling you to come visit her at Shedungent. There is apparently no time limit on when to go to her, just on delivering the Totem. Nulfaga will tell you who can receive the Totem (kind of like locking the barn door after the horses are gone) and teleport you to the entrance of the dungeon where you retrieve the Mantella to complete the Main Quest.