How about if the undead army of knights were led by the necromancer's wife...he betrayed her, you see, and had her executed for treason but her trusted manservant gave her the kiss of Life and brought her back. The soldiers of the undead army were a gift from her father upon her marriage to the necromancer, they remember her. When she appeared before them in all her undead glory, they changed their allegiances to a man, and tore the necromancer to pieces for his betrayal.
The other king is evil...maybe have the assassin brother start off in his army as one of his 'hidden daggers' (maybe this evil king uses his hidden daggers in the ancient rite of Decimation, to keep unruly soldiers in check), because he's disillusioned w/life, becoming enamored with the taking of it as it is a sacred art.
The blacksmith brother can be the armorer of the necromancer's army, but he became enraged when his lady, the necromancer's wife was slain out of hand and he took up a sword to avenge her- he was with the undead when they tore the necromancer apart. Since he is the only living member of this army, they use him as their representative to treat with the evil army (as they will not know they are about to fight undead warriors)...by chance his assassin brother is tasked to be the envoy for the evil king and together they devise a plan to put an end to the evil king and his loyal subjects once and for all.
When you have the climactic end game battle, have it being waged on a number of different fronts...for instance, write from the perspective of one of the infantry, you know, the horror of the bloodshed...in fact, make it one of the evil army's guys, some poor schmuck pressed into service because the only other choice was death to him and his family; then another perspective from the cavalry of the undead riding roughshod over their living enemies- maybe he could experience some sort of conscientious debate as he is not completely inhuman, the perspectives of the evil king and the undead queen - maybe they might wage war on the astral plane, or just mentally- almost like two people poised over a chess board and of course, the two brothers themselves, their unique perspectives should also be intermixed.
Just some ideas.