basically since the human side hatred against the demon lord is too strong, kanon realized that theres no way to obtain peace unless he kill the demon lord, so he asked for cooperation from the demonkind to change the demon lord name to avos dillheavia and spread it as the true name (they were against it at first but eventually agreed). Kanon would take up the name to then attack the human realm so that the human side will send their forces to the demon realm. Kanon would then pretend to be beaten by the human side. This way, since the "Demon Lord of Tyranny" is now dead, humans would feel no threat anymore and their hatred for the past 2000 years would be eased up. It took up the whole 2000 years so that arnos name would truly be forgotten by everyone, he was also hoping that humankind hatred would somewhat weaken but due to the magic of 1 character yet to appear in the manga, their hatred is as strong as ever.
Kanon is doing all that to protect his promise with arnos, he even tried to reincarnate multiple times until he gets reincarnated as demonkin, the current Ray.