In the Elder Scrolls lore Molag Bal did try to merge Tamriel with his own daedric plane. Also in the lore guess who went in and stopped it. Sotha Sil. We might get to meet him as a breathing entity and not a chained up corpse.

Sorry. Molag Bal destroyed a city not tied to merge Tamriel but he still did go into Oblivion and confront Molag Bal in the second era.

Molag Bal did it on the empires comand i belive. so that might not be related to the main storyline of teso

I read something about Molag Bal drawing Tamriel into Coldharbor being a plot devised by Molag Bal and the King of Worms. Other than that, I've not much knowledge about Sotha Sil making an appearance.

wait, have you taken this from the book called: 2920, Last Year of the First Era?

Wait a second. What you're talking about happende in the 1st Era (2920, to be exact) - time when all kinds of mad people summoned Daedric Princes to destroy their enemies, causing chaos and suffering. That was the time when the king of Anequina summoned Molag Bal to help him take revenge on someone who lived in the town of Gilverdale. That was also when Sotha Sil decided that enough is enough, and ventured into Oblivion to make a pact with Deadra Lords - from this day onward, they would only respond to sorcerers' and witches' callings, not to amateurs. He tried to avert disasters such as the ones that befell Gilverdale, when everyone and their mother could pit a Daedric Prince against someone they hated.

He did not confront Molag Bal per se. He talked to 8 Daedric Princes, and while we don't know what exactly happened there, we can safely assume it was more of a discussion than a battle.

A battle did happen later, in Mournhold, but that was agains Mehrunes Dagon, not Molag Bal. Sotha Sil's pact allowed witches to talk to the daedra, and one of them did - it was because of her that Dagon attacked Mournhold, despite Almalexia's and Sotha Sil's efforts to stop him. They managed to cast him back into Oblivion, but not before the city was destroyed.

None of this has anything to do with the storyline of TESO. However, in the 2nd Ear Sotha Sil is alive and kicking, so I don't see why we shouldn't meet him - apart from the fact that he's not the most sociable fellow. Maybe we'll get to talk to him, though