Necromancer... a wizard, black magician, specialized in dark arts of death, reanimation and drain vital forces.
For me Necromancy is splitted into reanimating dead corpse back to un-life, corrupting and/or depleting enemy life and directly decay and destroy flesh. So lets go into each of this:
1) Master summoner - Main abilities are reanimate (a.k.a. Summoning) and command. Starts with reanimate nearest cadaver that can only craw and bite but with leveling skills of caster, minion retains abilites to walk, run, use its original abilities and etc. Next base-line ability is summon spirit from recently died creature (Need to be adjusted against world's lore - does all creature have souls or only some types, ex. Humanoids). Spirit-type summons also can be developed into more powerful types. With developing of Necromancery's summoning skills, the Necromancer gains abilities to creature different type of undead craetures to deploy in combat. They had to be prepeared at some point before and will cost some time and resources but can be customized in some manner. Here comes cadaver types as Mummies, Draugrs, Jiangshi and etc. Culmination of Summoner development is patchwork similar to monster of dr. Victor Frankenstein (or WoW's abominations). Number and super-types (cadavers, spirits, skeletons, etc.) of undeads that can be deployed of any one point depends of Necromancer's skills, and their actual type (cadavers: zombie, ghoul...; spiritual: phantom, ghost, wraith, and etc) are based of what is prepared before. They can be switched with restriction (Only out-of-combat || with CD's || as single-use-item || something else). As side skill can force spirit of fallen comrade to return back into the body (acting as combat ressurection).
Skills: Reanimate Body(as gradation over Cadaver, as keeping some its original abilities), Recall <Spirit>, Capture Spirit (Capture spirit ot recently slayed enemy for further use//summon), Summon <Undead>
2) Necrolyte - Concentrated over affect body and soul of the target as doing direct damage or weakening some or all of its abilities. With advansing of this path more poweful spells become available conventional mage. Beside the Necrolyte uses Dark arts and Death magic.High in developing is strong corruption magic that destroy will and mind of target non-ellite || non-boss || non-immune turining it into mindless minion that slowly dies. At the end of development some of spells helps Necrolyte and allies to regains lost health.
Skills: Flesh decay (slowly destroying enemy's flesh causing direct damage), Aging (slow enemy reactions), Unlearn (block/disable enemy ability), Soul rend (direct damage), Rust (decay enemy weapons and armor, reducing their values)
3) Grim Ripper* - Focused on directly absorb enemy vital forces in close combat. Kinda vampire. Ripper's knowledge of anatomy allows allows precise strikes in vital points of enemy and magical skills transfer life from victim. However such lifesteal do affect the receiver. With developing more sophisticated and skilled manners of draining life, Ripper gains some abilities that are fueled by victim's life force. Failing to provide it leads skill to become dormant until its refueled. There is array of different skills that can be activated based on Ripper's development and number of parallelly active depends both on Ripper's development and on amount of currently available drained vitality. Close encounters with death physically strongly affect the Ripper drying flesh and turning it into deformed skeletal corpse (Read as: Fantasy Lich) with some benefits as (almost) undead. Depends on developement, Ripper can be turned into melee combatant slash healer for party (cannot distribute life force to too many people) || Self-sufficient tank that draing heal || Melee combatant
Skills: Kiss of Vampire(directly transfer life), Flesh of my Flesh (Mark friendly target to receive drained health instead Ripper), Blood fontain (when 'Bleeding' Status effect is applied, neares X allies recieve health), <Something with bleeding status effect>, Necrotic touch (Melee stricke taht apply 'Flesh Decay')
-> Additional skills (maybe) non-developement based:
a. Create Phylactery - create an artifact that holds Necromancer's soul. Can create multiple and scatter accross the world. In case of death, Necro can self-ressurect at (any of) nearest one.
b. Reanimate cadaver - bring death corpse to (un-)life with very limited abilities.
c. Recall Spirit - Force spirit of slain enemy to fight for Necromancer for limited time (can be used out-of-combat for some Class-quest or something).
d. Dead helper - small minion created by necromancer. Much small and weak version of Master Summoner's Patchwork.
d1. Grim Harvest - out of combat - ability to collect body parts to modify Dead helper from d.
e. Dust to Dust - Destroy enemy minion by sacrificing own minion
f. Hungry dead - reanimate any corpse(s) (Number is based on build, level and development) to attack nearest target no matter friend or foe. Immobile.
g. Bone Melting - Reduce combat effectiveness of target. May insta-kill skeleton-type enemy.
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* Y, Ripper as Jack the Ripper, not as Grim Reaper....