The story ends with the final battle of Camlann in which Arthur's forces confront Mordred's. The outcome of the battle will depend on the number of combatants on each side, and that in turn will depend on the decisions of the nobles as to which side each will support. What factors will influence this decision? The primary one should be "kingly stature", the extent to which Arthur is perceived to be a leader of strength, integrity, and power.
But should other factors enter into the equation? Should personal anima towards Arthur enter the decision? Suppose, for example, that Arthur has had a fling with a female noble, a fling that ended badly, but otherwise has behaved well. Will she go over to Mordred over the failed affair? That leads me to suspect that the three basic relationships (affection, trust, submission) should form the core of kingly stature. Perhaps commitment should enter into the equation as well.
The next question is, should Mordred be tested by the same formula? Do people compare Mordred's stature with Arthur's? I suppose they must, but this seriously compromises Mordred's ability to engage in dirty tricks. Perhaps Mordred should have some sort of immunity factor to the emotional
repercussions of dirty tricks -- people turn a blind eye to his actions, figuring them necessary given Mordred's position.
Thus, Mordred's strategy is primarily to diminish the aggregate values of affection, trust, and submission toward Arthur. This gives us another handle on the problem of conflict behaviors: how does Mordred get people to reduce their values of affection, trust, and submission to Arthur?
The way to reduce affection is to arrange for Arthur to injure them, or to create the impression that Arthur injured them. One way to do so is to force Arthur to render a judgement against a person. Another method is to encourage a lady to attempt the seduction of Arthur. If her attempt fails, she'll resent Arthur, and if it succeeds, it can be put to even more mischief. The impression of injury can be created by false gossip; this in turn will require a set of verbs by which a character can yak about another's personality behind his/her back. By falsely putting such words into Arthur's mouth, Mordred can create the impression of injury -- so long as he's not caught. Of course, who'll do the catching? Arthur?
To reduce trust, Mordred must create scenarios that make it appear that Arthur is lying or reneging on his deals. This can get pretty intricate, but perhaps Mordred could go to people demanding "Who told you...?" regarding crimes that he really did commit, but implying that the reports of these crimes are lies. He could then accuse Arthur of lying. Perhaps. A more effective approach would be to advance the hypothesis that Arthur is carrying on an affair behind Guenevere's back.
To reduce submission, Mordred must get Arthur to do badly against the Saxons, either by refusing to fight them often enough, or failing to engage them in battle, or losing in battle, or bringing back few spoils when he is victorious. This would discredit Arthur in the eyes of those who support him militarily.
Another avenue concerns the Grail, which Mordred can deny as a propaganda ploy. Arthur is then put on the defensive, trying to prove its reality by getting Percival to find it, but Percival is reluctant.