Here is the updated description of the necromancer.
Height and Length: Same as a wither skeleton
Health: 60 points (30 hearts)
Death and Decay: Shoots an orb of death or summons a rune that reduces it's victims life and heals the caster (like a wither effect).
Rise Of The Undead: Summons a small group of undead (2-5). The undead that you summon attack all mobs that you attack. As soon as they get hit by direct exposure to sun light, they instantly die.
Soul Swap: The caster swaps it's current location with an undead mob he chooses (with a right click).
Ally Of The Undead: The player that has the Necromamcy effect is not attacked by undead mobs (summoned or not summoned).
Soul Collector: Each time you kill a mob or an other player, you collect 1 soul that is automatically used to fuel "The Totem Of Undead" (Necromancer item)
All active abilities and passive abilities are usable while you have the necromancy effect (except when you have soul fuel on your Totem). All abilities listed are used by the necromancer (except Soul Collector) These abilities can be used by the player as well (including Soul Collector) while having Necromancy or a Totem with soul fuel.
The Necromancer can be found on a Necromantic Tower in a swamp. The tower is medieval looking with spider webs and iron bars. Surrounding it is a small graveyard.
Once you encounter the necromancer face to face he will display a message saying: "Join me or join the dead". If you click "Serve" you can bring him 1-3 wither skulls (randomly choosen by the necromancer) and he will award you with 10 Minutes of Necromancy and a full fuelled "Totem Of The Undead". If you don't accept his offer he will engage you in combat. Using his necromantic abilites he will try to kill you. Once you kill him you only get his robes (or armor, depends on design). So if you're looking for some dark powers be sure to ally with him.
The Totem Of Undead is a special item that can be used only by Necromancers. The moment you recive it for the first time it comes full of souls. When you use an active ability, it consumes souls from itself until it gets empty. You can refill it by using the passive ability Soul Collector. Every active ability has its costs of using (ex Death and Decay uses 2 souls per usage) The totem's max capacity is 25 souls. The totem cannot be removed from your inventory unless you die, and when you do die; the totem gets full on souls. Per each mob you kill you gain 2 souls and for each player you get 5.
I have a design that can be used as base but it even can look as the classic Necromancer with dark robes and has a skull for a face.