
Claxon |

Actually, part of the point of it is that no one except Asmodeus and the deities that signed the document are actually aware of what it grants.
It is a central plot point in the lore that has not yet been explored.
Presumably because while the deities are bound by contract not to interfere with some of Asmodeus activities per the agreement (most likely) mortals would not be bound by that agreement and if they knew the content and context of it would be able to subvert it.
Some popular theories include the idea that Asmodeus was granted domain over souls that acted in an evil manner, but went further by purposefully corrupting people to gain dominion over their souls. The contract also giving him the right to use the energy of the souls to grant him increasing levels of power to raise him to a similar power as true deities.

Bob Bob Bob |
Alright, so for information on devils you want Book of the Damned, Vol. 1, Princes of Darkness. It includes the original schism (which was actually between Law and Chaos) between the First of the Seal Ihys for free will and Asmodeus for order. The contract of creation is mentioned only in passing as the deal they made after Ihys was dead and Asmodeus had decided to let free will survive as a last act of pity to his brother. It honestly sounds like the contract of creation here is just some kind of agreement between the gods not to make new mortal life and probably not to interfere too much with the mortal life that already exists. Maybe not to create new morality (heck if I know what that would be, but the creation of free will (and by extension chaos) was what kicked off the first war of the gods).
Personally I prefer one of the Fiendish Codex II versions from 3.5 where Good or Evil were less important than Law versus Chaos and Asmodeus was a servant of the Lawful gods who was created to fight demons. Over time him and his followers grew more and more brutal but when the gods tried to cast them out he successfully argued that he'd never broken the law. Then when the gods made mortals the demons attacked them too, the gods threw up barriers but the mortals kept finding ways around them and unsealing the demons. They didn't know why the mortals wouldn't listen and Asmodeus told them it's because there was no reason to, they had no concept of punishment yet. So he got himself and his friends appointed to punish the sinners, eventually the constant torture in heaven convinced the gods to give him his own plane so they wouldn't have to hear the torment of the sinners. They weren't willing to make him a god so he argued they needed to let him use the sinner's souls to power his magic. In order to make sure this was all above board they signed a contract giving Asmodeus full rights to "wicked" people and the power to use their souls to fuel his powers. Over time they noticed less and less souls going to the upper planes and so investigated and found he'd turned his personal plane into a horrific torture chamber that spawned more devils to go tempt mortals to sin. When the gods tried to stop him he pointed out they should have read the fine print.

Xyden |
Here's the deal. According to the entry in the pathfinder wikia,"it was he who is credited with the penning the contract of creation, within which his followers believe is hidden the means for their patron's eventual rise to supremacy." So my lawful evil warpriest will be trying to accomplish Asmodeus' rise to supremacy. That's why I want to know the text of the contact.

Claxon |

Xyden,
You're player can try. But while Asmodeus' followers believe that it will lead to his supremacy it needn't necessarily do so. It fits with what followers of Asmodeus should believe. Because he promotes using hidden knowledge and contracts to your advantage and to gain dominion over others, but that doesn't mean the contract does that. Or that a mortal would have any conventional way of using this knowledge.
Hell, it could just say in a round about fashion that Asmodeus gains dominion over all souls (not otherwise dedicated to a god) when the die as a means to eventually gain greater power than the other gods. Not much any worshiper of Asmodeus could do except to promote atheism or worship of Asmodeus.