Open to suggestions for what types of Augmentations and Dirgecastings should appear. The route I'm going for is "Augmentations make him decent at a bunch of things," and "Dirgecastings should level the playing field, either by making your enemies weaker directly, or by increasing the power of your summons."

Summons is going to be straightforward Animate Dead / crafting with HD = (CHA+2)xCaster Levels, stacking with Dread Necromancer if multiclassed.

Proposed Mechanic for Augmentations - give a bunch of very specific options, the bonus for each is equal to the number of augmentations you've got.

Proposed mechanic for Dirgecastings - Performance-based with a Wide variety of options, each choice adds a layering effect to your performance. Performance affects your summons and your servant, and possibly yourself. Effects can stack, but you've only got 5, so choose wisely.

Spectral Orchestra - your summons can continue (or bolster?) your Dirgecasting

Possession - You join with your Abysmal Servant for a short time, gaining bonuses based on its formation choices and your augments. Should be interesting.

Howling Concerto - boosts your dirgecastings

Wailing Crescendo - adds debuffs to your dirgecastings that affect opponents

Gruesome Instrument (Was gonna name it Soul Choir, but that could be racist.) - Your summons are now the only instrument you need for your grim performance. Possible Fear Aura while performing, can potentially panick, automatically gain ranks in Perform (Undead Horde) equal to your highest perform skill's ranks, all bonuses to that skill also apply to perform (Undead Horde) - basically, Perform Undead Horde becomes as powerful as your current perform skill, and gains bonuses depending on how many HD of undead you perform with. Dirgecastings range is measured from the 'edge' of the area of your horde, possible directed sonic attack with increasing power based on how many HD of undead contribute.

Master of Death's Symphony - A swarm of ethereal spirits temporarily do your bidding. Should have oodles of non-combat abilities, is limited by CON and CHA drain, and basically just sits there being another badass endcap that makes you want to take this thing all the way to 20.

UH. Yeah gonna be some definite balance issues if any number get too high, but if I can get it right and if a lot of the abilities create a good choice-pool for individual feel and flavour, I can see this being pretty cool.