Simple give them a buff, like berserk for warriors, to increase damage and lower defense.
With a base of GLA the job could go like....
1)Dark Pack
-Greatly increase base weapon damage
-Negative side effect like constant damage over time on user or greatly increased damage taken
-Toggle buff like berserk and rampage
2) Infernal Strike
-WS Decreases enmity when Dark Pack is up
-Chances in place a DoT on the enemy that heals you for damage dealt by DoT
3)Dark Wave
- Spell that does AoE umbral damage at the cost of life
4) Eye for an Eye
-WS damage increased on lower life of Dark Knight
5)Unholy Sacrifice (15 min)
- Takes Dark Knight to 1 HP. Deal Massive AoE Damage.
If tanking, keep Dark Pack off. When doing damage turn dark pack on and go to town.
I picked two WS since the gla lacks spam-able damaging WS, like other damage classes.