This will be a source of dilemmas for Arthur as well as a distraction. Feuds are conflicts between actors.
Sources of feuds
Romantic relationships. People are constantly engaging in minor flirtations, which could (depending on the inclinations of the actors) blossom into romantic relationships. These relationships can be a source of feuds if one of the partners already has a commitment (however low-level) to another actor. This third party can then initiate a feud with either of the two new partners.
Mordred seeks to initiate feuds between Arthur's closer followers. He certainly does not want to initiate feuds involving his own followers. Mordred initiates such feuds by telling lies. Lies can suggest secret affairs or third-person insults. Trickiest would be lies about lies: e.g. Mordred tells Lancelot that Galahad told him (Mordred) that Lancelot is having an affair with Guenevere. This could get pretty complicated to keep track of; for now, I shall defer action on this option until I am sure that regular lies are working well.
Prosecution of feuds
Once a feud has begun, it is prosecuted in an escalatory sequence of injuries. This will be much like the sequence used in the earlier LMD. A feud can be terminated in several ways: one party can decline to continue the feud, Arthur can informally intervene, and one party can bring charges against the other, in which case the justice system is triggered.