Oh, forgot to mention: Molag Bal is presented as a serious threat at a time when ALMSIVI was at full power. I'll remind you that Almalexia and Sotha Sil single-handedly beat the living crap out of an avatar of Mehrunes Dagon after he destroyed her city.
This is correct.
Let me repost something I printed adressing this:
'I can see it fitting his sphere. If he had a plan for it to work that is consistent with current lore on the topic, the most important barrier in his way is the Daedra pact with Sotha Sil by which Daedra lords only heed the summons of powerful wizards or witch covens on auspicious days.
There is also the (largely apocryphal, though not completely so) lore of the towers that hold the world up and protect it from Oblivion.
If the Lord of [laughing ponies] finds a way to circumvent these things, possibly by a subversion of the literal rules, hes not invading but swallowing, or allowing Nirn to drift into Coldharbour by something mortals did, then that would be right up his ally.
I think.
Edit: Another possible barrier for Molag Bal is the Triune.
If Dagoth Ur still sleeps under Red Mountain at the time this is set, theyll be at the height of their power.
If its set after he has awoken he will have done so recently and the Triune will have only missed one renewal, and would still be active gods.
Vivec wont like his people being swallowed by a god he already defeated once.'
The story is that Molag Bal is trying to svck Nirn into Coldharbour, similar what happened tohttp://www.uesp.net/wiki/Lore:Tal_Marog_Ker%27s_Researches.
Maybe he got the idea from there.
He is prevented from acting on Tamriel, but does that also mean that he is prevented from swallowing Tamriel into Oblivion?
This subversion of the literal rules that prevent Daedric intervention, this abuse of a loophole, fits his sphere perfectly I would say.