With the necromancer / dark magic in mind, I think of life-force manipulating abilities. They drain their enemies life to replenish their own, they sacrifice their own life to damage their enemies/buff their allies, and maybe they are able to briefly animate a fallen foe/ally as a ghoul to help them in battle.
My primary concern is balance. In EQ, it seems like SKs can be squishy. In WoW, Death Knight’s healing is too strong, and they can accomplish feats no other tank can due to their absurd self-healing.
To counter the above, I would do something of a blend. Allow SK’s to be very sturdy in combat, and allow them to manipulate their own life total with some strict healing limitations and/or have their allies healing magic not play so nicely with the SK’s own dark magic in the form of SKs taking reduced healing from non-SK allies. I have no idea how that could work in practice, just spit-balling.