I don't think this is extensive or identifiable enough to qualify as mantling. It's a single action, and a rather mundane one at that ("kill this dude in my stead"), rather than the long process of play-pretend that constitutes mantling.

Is not and never was a long process of play-pretend.

Ulfric: Yo, Tullius, you're so far beneath my notice I'm not even going to fight you myself, I'm going to sic my Dragonborn of Mass Destruction on you. And then I'll give the Dragonborn my sword and all of history will remember how Ulfric Stormcloak's blade was the one that purged the foul Imperials from Skyrim's soil! It'll be epic, I tell you.

Theoretical Dragonborn: Erm, that's probably not the best way to convince the Empire to actually drop the conflict; you're still gonna have to make peace with these people ...

Ulfric: EPIC!

Tullius and Theoretical Dragonborn: (Exchange long-suffering looks)

Theoretical Dragonborn: Ah well; let's get this over with. (Proceeds to pound Tullius down until he's crouching down, wishing he had gone up against half a dozen dragons instead)

But yeah, I can see Ulfric having gone bad long before this.

Loranna

This strikes me as a bit reaching, or even accidentally falling into a mephalian web. Swearing vengeance or stopping at nothing to get revenge isn't the same kind of thing Bal does to people. None of that is really oath-breaking in and of itself, or some powerful transgression against their own character, and isn't motivated by any sort of personal assault by Ulfric. YOur hypotheticals strike me as the logical consequence of Ulfric stumbling into the string of one of mephala's webs, the detonation of a hypothetical powder keg. However, when it comes to Ulfric's treatment of Tullius, it's worth noting that only three people ever saw what happened (and lived). A player character, Ulfric, and Ulfric's loyal general. No soldiers are going to know what really happened there, all anyone will know is the story Ulfric tells. Another thing is that Ulfric spared Elisif. While the Jarls and Thanes of the various other cities might want to martial for war against Ulfric, the fact is that they were all in Solitude during the final battle, when Ulfric claimed the city. Everyone except for Elisif and her thanes have been effectively captured. As for Elisif herself, she's always been a puppet. She has no strength or vigour in herself, and her naive incompetence was long tempered by her thanes. With her fealty sworn to Ulfric, the nobility of Solitude are neutralized. I wouldn't call it an alarming trend. Ulfric comes off from the beginning as someone whose underhanded in fighting, and who uses all the techniques available to him. He can't really be said to get worse either, and his character remains pretty stagnant during skyrim itself. I don't really see it happening. Ulfric's a dirty fighter with high political ambitions, that's much more in line with Boethiah then Molag Bal.

@loranna, when choosing text for a link, write out the text first, highlight it, then press the link button in the editor to create the link.

Is your problem with my word choice, or with the idea that it is a prolonged process rather than a single action?

Thanks for the tip on linking; I knew it was something obvious I was missing.

Regarding Ulfric falling more into Mephala's or Boethiah's purview, I wouldn't argue against either of those. My line of reasoning was "Monstrous act begets more monstrous acts, as the King of Monsters begets new monsters" - an interpretation of Molag Bal which is largely my own, based on trends I've heard of for the abused to become abusers in turn. It's also quite possible, I'd say, to look at Ulfric's actions overall and put him in Mephala's or Boethiah's sphere - the Daedra Princes are wont to step on one another's toes at times. Hell, with all the civil war and killing going on, you could argue that Ulfric is supporting Mehrunes Dagon's sphere of influence too!

Loranna

It's story-based, and event based. Everyone has been taking one statement of Michael's far too literally.

Tiber Septim: "The Stormcrown manted by way of the fourth: the steps of the dead. [...] walk like them until they must walk like you.

Really, though, my problem with this theory is that the event isn't in any way characteristic of Molag Bal. It's just two quests that use the same animation and a similar pattern. I suppose you could draw parallels in terms of literary criticism, but I don't think they are enough to move it into the realm of lore. None of Molag's identifying characteristics (his violation of mortality and human kind via the creation of vampires, his seduction of Vivec and the monsters resulting from that union, his spheres of graqe and scheming, etc.) are present in the act, or in Ulfric. My Dragonborn does the same thing anytime she beats down a cowering bandit begging for mercy.

To put it another way: me posting a Cyrus story doesn't give me the chance to become Michael Kirkbride.

It's more because he knows that he's making history and he wants it to be epic. In his mind, it makes for a better song if the Dragonborn delivers the killing blow (regardless of whether or not it is Tullius or himself being struck down).

(Yes, the previous statement assumes that Ulfric is in this whole 'become High King and start a new legend' game for the sake of his own ego, rather than the good of Skyrim. YMMV)