A powerful magically binding legal document that establishes rules of the world. The secrets, power and agreement necessary to create a concordat means it happens very rarely.
Many details of old concordats have been forgotten, resulting in the need for experimentation and archaeology to rediscover rules, exceptions, clauses and possibilities.
The Draconic Concordat is agreed to be the latest true Concordat, and is the year at which the current time estimation is counted from (ADc - Ad Draconic Concordat.)
Some use a more open definition of concordat, to not leave out the number of smaller but still meaningful magically enforced agreements or systems in the world.
Concordats have often been written on runestones or monoliths, of special materials conductive to magic and created to endure. Some of these have stood the test of time, whereas others have been relocated and gone missing/been stolen, some have worn out and been grown over and forgotten, and others again have been swallowed by the earth or ocean.