1. No battle shall be fought with more than 1,000 soldiers per Clan.
2. Respect that which has gone before.
3. Do not practice blood magic.
4. Bow to the Celestial Order.
These are the decrees of the Emperors past that still hold sway today. Following these edicts is a strict point of Honor. Assuming that they are never broken is more naive than a Great Clan Daimyo can afford to be.
Edict 1: the most often broken law. It is (sometimes) a hard choice for a daimyo to make the decision of what is most important: following this law and letting a rival clan build that monastery/fortress on your borders or making sure those dishonorable dogs never get the chance. What if another clan tries to annex your lands by marriage? (Or blackmail?) What if they invite your clan’s greatest duelist and most noble poet to their winter court and they never return? Some insults must be met with as harsh a punishment you can afford. The honor of your ancestors demands it, even as your loyalty to the Emperor demands you must not.
Edict 2 tightly circumscribes innovation in social practices, crafts, magic and religious practices and war. Things have moved very slowly in the Empire for the last 10,000 years, and that is just the way the Celestial Order wants it. It is far more honorable to learn the old way of doing something, the way of your ancestors, and perfecting it that thinking up some clever little trick that makes you more efficient. Clans who hold efficiency above honoring the way of their ancestors will not be thought of highly by their neighbors.