Lodoss-tou Senki: Seiyaku no Houkan

The story takes place 100 years after the main events of the original series and follows Lyle, a prince of Marmo. To preserve peace on Lodoss all rulers were given a diadem to which the kings vowed not to engage in war. An heir in Flaim threatens to break the covenant.

(Source: Record of Lodoss War Wiki)