According to UESP, the Compact was agrees on the 5th of Rain's Hand, 1E 2920, the last year of the First Era, after Molag Bal manifested and destroyed the Wood elf village of Gil-var-delle, which you can actually visit in Grahtwood.

No mortal knows the exact terms, even into the Fourth Era. In broad strokes, it seems to limit most physical manifestations and interactions on Nirn of the Daedric Princes that agreed to it, originally eight (Azura, Boethia, Hermaeus Mora, Mehrunes Dagon, Hircine, Malacath, Morag Bal, and Sheogorath) with Mephala and Clavicus Vile agreeing at the end of the Summerset story. That leaves Nocturnal, Vaermina, Peryite, Namira, Sanguine and Meridia as Non-signatories unbound by it (which would explain things like Nocturnal and Sanguine showing up in-person in Skyrim.) Mehrunes Dagon tried to break the compact almost immediately later in 1E 2920 to sack Mournhold, only to get curbedstomped by Almalexia and Sotha Sil.

The only clue as to how Sotha Sil managed to get these princes to agree to the original compact is Secrets Overheard in Apocrypha, which might also explain why events such as the Planemeld and the future Oblivion Crisis are possible: Sotha Sil apparently offered *something* of such value to Hermaeus Mora that he agreed to a pact with all of Nirn to not interfere. But that this pact goes both ways, and mortals are technically not suppose to visit or go romping around in their realms of Oblivion either. It may be that while the Daedric Princes who have agreed to the Compact cannot mess with Nirn directly of their own volition, the actions of mortals could be the loophole they need to still mess with Tamriel, especially when Akatosh's own deal with St.Alessia to protect Nirn is in limbo when the Dragonfires aren't lit. After all, every Dolemen event begins with Worm cultists preforming a sacrifical ritual to Molag Bal. With regards to Dagon and the Oblivion Crisis, it could also be that with the Tribunal all dead or missing after the events of Morrowind and its expansions, there is noone left to enforce the terms.

We might not for sure the full agreement, but one thing is for sure there is reasons for the Princes to Fear him like they do Jyggalag.
In the Shivering isles dlc for Oblivion. There is this one quest depending what duke you replaced you get sent to help the golden saints or seducers. The idea is when Thaden/Syl enter they let in Order. Order cuts off the chaotic creatia completely and kills off half of Sheogoraths army by turning them all to stone or crystal or something. His power is enough to seal and destroy the Daedra and the only way it was undone was to break the seals his forces put into place. Hes the one force that can do this and we see this in Oblivion Shivering Isles.
The Power of Jyggalag and likely his essence is the pure opposite of the other Daedra. He is not only Order he likely runs on his own Orderly Creatia. This Orderly creatia could be the bane of chaotic creatia. It can outright seal/destroy it. There had to be a good reason why all the Daedric Princes worked together to curse him. I feel that is the reason and Sothia Sil, comes along now he creates this Clock Work city has many Jyggalag Principles and its designed to be the potentially future destruction of the Daedra but Sothia Sil in theory likely knows that until the new world is ready to be born or whatever he just can't seal up important forces so he made a deal with them not destroy them in return for certain things some might have gotten better deals then others.

The Daedra rely on Order, even as beings of chaos and you can see this clearly in how they work. But to much order and it can fall apart and destroy them, to much chaos and it could rip apart their realms potentially as a theory as they would revert to the states of formless chaos. So to much of either is very bad for them. There is a reason why Jyggalag is allowed to come back to destroy Sheogorath and his realm. Its a cosmic balancing act. It keeps Sheogorath from becoming the threat to the status quo that Jyggalag was and Sheogorath exists to keep the Daedra safe from Jyggalags Orderly Creatia. Sothia Sil likely knows this and its clear that in the Sermons there is hints that talks about Jyggalag though its likely bias but there is likely some truths there.
Sothia Sil if he had the ability to manipulate a form of Orderly Creatia, and had the power to do what Jyggalag would do that would threaten Oblivion so hes a wild card that the Daedric Princes likely cannot access his realm very easily. They do know hes a threat to them and for a Mortal that is pretty impressive.
They took whatever deals he offered and well I'm sure one of those things was not to destroy them. .
Sothia Sil if his Clock Work City could seal the gaps, and reorder things, then one of the things he could do is reorder Sheogorath. It would be one way in theory of ending Jyggalag's Curse and not only that make him a threat to the Princes again but Sothia Sil could also likely use his city to do the same thing to the other Princes. So the idea would be that he can seal up the gaps and the effect would likely look the same as the golden Saints or Seducers when their wells are sealed off.
So that is what I think Sothia Sil offered them. He offered not to destroy them in this manner.

So we will never know for sure what he offered but logically given the lore and what the Clock Work city could potentially do to the Princes. I would say it puts him on a serious threat list to the existence of the Princes. Which is why when the conspiracy was ended, Two of the Daedric Prince's in the Summerset Conspiracy were so willing to join the compact. So what he likely offered them is to allow them to continue to exist. That is I think the main part of the compact. So there is likely a bunch of stuff to the compact but I think the threat that Sothia Sil Poses to them could be the biggest reason they agreed to it. Dagon didn't invade in full till after the events of Morrowind so I think he waited till Sothia Sil was dead to start the Oblivion Crisis in full.

I'll always figured that Jyggalag's power in the Shivering Isles had more to do with his being the lost aspect/personality of Sheogorath; he has the power to do all that because technically, it is *his* realm, and the realms of Daedric Princes are as much an extension of the Prince who resides there as a physical place.

Though Shivering Isles does have another hint to the terms of the compact. When you enter the portal for the first time and talk to Haskill about the portal, he'll say something like, "...No compact has been violated, it is a doorway, an invitation..."

Considering that Sheo went with a massive friggin' archway addorned with screaming heads, it suggests that the compact allows for a lot of clever leeway as to how to "invite" mortals into getting involved with the Daedric Princes.