Marut MPMM p173

The nigh-unstoppable inevitables serve a singular purpose: they enforce contracts forged in the Hall of Concordance in the city of Sigil. Primus, the leader of the modrons, created maruts and other inevitables to bring order to dealings between planar folk. A wide array of disparate creatures, including yugoloths, will enter into a contract with inevitables if asked.

The Hall of Concordance is an embassy of pure law in Sigil, the City of Doors. In the hall, parties who agree to mutual terms—and who pay the requisite gold to the Kolyarut, a mechanical engine of absolute jurisprudence—can have their contract chiseled onto a sheet of gold that is placed in the chest of a marut. From that moment until the contract is fulfilled, the marut is bound to enforce its terms and to punish any party who breaks them. A marut resorts to lethal force only if a contract calls for it, if the contract is fully broken, or if the marut is attacked.

Inevitables care nothing for the spirit of an agreement, only the letter. A marut enforces what is written, not what was meant by or supposed to be understood from the writing. The Kolyarut rejects contracts that contain vague, contradictory, or unenforceable terms. Beyond that, it doesn't care whether both parties understand what they're agreeing to.