Limerance Dogma 1
"... and so the four knelt to King Alar of House Grayson, with Queen Sugan of the West kneeling first, followed by Queen Valeria Redrain of the North, then Grand Duchess of the New Lyceum Procella Pravus, and finally King Aposyr Thrax of the Mourning Isles. And each then spoke as follows: 'Before the gods, I swear my life, my house, and my sacred honor to our Compact. I swear that when the Compact calls me to war, I will answer. I name Alar of House Grayson my king, and swear my leal service to him.' And thusly, the Compact was born and the five kingdoms made one. For the four had made oaths before the gods, and to break that was unthinkable." --- Dominus Tin, 'A recollection of the Forging of the Compact'
In the Compact, giving one's solemn word is never done lightly. To make an oath before Limerance and then knowingly break that Oath is unthinkable. It is trained from a very early age, including in the Lyceum, that someone's honor is completely reliant upon their fidelity to their sworn word. Someone that willfully breaks an oath and becomes an oathbreaker is stating to the entire Compact that honor has no meaning to them and they can never be trusted in any meaningful way.
This doesn't necessarily imply any sort of criminal penalty for becoming an oathbreaker, as a great many oaths, vows, promises and pledges are done over matters that fall far short of criminality, but merely represent someone's commitment. Nonetheless, someone that is clearly an oathbreaker is effectively socially dead as far as the Peerage is concerned. It reflects extremely badly on a Peer of the Realm to even associate with an oathbreaker, and it is all but impossible for a known oathbreaker to rule effectively if they are the titleholder for a demesne. If the Faith of the Pantheon declares someone an oathbreaker, that level of censure falls just short of excommunication. No one is obligated to keep their own oaths to an oathbreaker, as an oathbreaker cannot appreciate or reciprocate that level of commitment.
All contracts, arrangements, and most laws in the Compact are essentially guaranteed by the honor of the individuals involved and their fidelity to their sworn word, often recorded on on Vellichorian vellum. Breaking one willingly, and being declared an oathbreaker, makes it known that the oathbreaker is incapable of honoring these, and therefore no one is under any obligated to honor them in return and risk being cheated by them. Thus it becomes nearly impossible for an oathbreaker to perform any kind of business in nearly any way.
Quite often a succinct missive from a legate, reminding a reticent lord that an action could label them an oathbreaker, is more than enough to break any sort of acrimonious negotiation.
A great many of Abandoned clans after the Reckoning were created due to oathbreaking, when an entire house performed an action that was judged in violation of oaths to a sworn liege and excommunicated and declared outlaw. Even aside from the years of blood between the Compact and countless Abandoned clans, this stands as a reason why so many in the Compact are slow to trust prodigals, for the implication that honor may be foreign to their nature and they come from a culture that tolerates oathbreaking.