Concept as of 11/28/2020

Spell Casting by Proxy
In the movie, “The Crow”, the undead protagonist is invincible if his agent to the other side, the crow, is healthy. Hurt the crow, and the protagonist can be stopped. The crow basically can’t do anything on its own. His agent, the protagonist, does the work. Can this be replicated in Grim Dawn?
What if the necromancer must learn which agents of the dead to master in order to achieve the desired outcome? Instead of the necromancer casting the spells, he calls forth a proxy to cast them. For example, the necromancer summons a rat and the rat summons the skeleton. Testing has revealed a problem with this approach. If the 2nd generation pet (the skeleton in this example) makes the kill, the necromancer doesn’t get the XP. This method is fine if the source of XP isn’t from kills but in the main game, that’s where it comes from.
What if the necromancer summons the skeleton and the skeleton summons the rat? The initial summoning order doesn’t really create the experience that’s desired since it’s backwards. Also, the skeleton can’t be made vulnerable or destroyed by the death of the rat. It does help establish the theme though.

Vinculum (Proxy)
Familiars : Lich
Hounds : Ethereal Undead
Insects: Spells that drain or debuff
Rats : Skeletal Undead
Ravens : Fleshly Undead

Undead Lords
If you take all of the skills associated with one particular vinculum, you will also have many (if not all) of the abilities of an undead lord. The familiar line will turn you into a lich, the hound line will turn you into a ghost, the insect swarm line will turn you into a plague lord, the rat line will turn you into a death knight, and the raven (wish I had bats) line will turn you into a vampire.
I don't currently see a way to prevent the player from taking more than one of these abilities. It could result in something bizarre like a death knight ghost. I guess if that's the experience you're going for though then so be it.

Undead Pets
If you've played the standard necromancer, I'm sure you've had the desire to pick the type of undead unit you want summoned. To that end, I've been inspired to take Raise Skeletons and break it into four separate skills.  All four vinculum lines will have their own equivalent of warriors, archers, mages, and knights. This is not just an aesthetic difference. There will be stat and skill differences as well.
From my research, I've not been the first to want to limit the number pets through an energy reservation but that doesn't seem to be possible. It looks like the most skeletons you have have with a standard necromancer is twelve. It is for that reason that I have picked twelve as the limit for what a skill may animate or summon. I see no way to limit the total number of pets between all abilities. That means that you can have twelve of each type of undead: skeletal knights, skeletal warriors, skeletal archers, skeletal mages, ghost knights, etc. This limit then will be determined by how you want to spend your points and if your PC can handle all of the extra demand.

Damage Types
To fit the theme and my own sense of lore, each vinculum line will focus on a different damage type.
Familiars : Fire & Lightning
Hounds : Aether & Cold
Insects : Chaos & Poison
Rats : Physical & Piercing
Ravens : Vitality & Bleeding

Vulnerabilities
To fit the theme and my own sense of lore, each vinculum line will have their own vulnerabilities to a different damage type.
Familiars : Fire & Physical
Hounds : Fire & Lightning
Insects : Cold & Fire
Rats : Fire (I'd like to have acid too but acid & poison are lumped together in the same resistance. I've made undead immune to poison)
Ravens : Fire & Piercing