Attacks with magic, stays at a distance and runs in red health.
Pretty easy to kill because of their low health but can be annoying if not killed quickly as they become invisible and use multiple spells so they might be a problem for inexperienced adventurers. A Dwarven Ring may be helpful while hunting them. In update 8.4 the invisibility length was increased making voodoomasters more of a challange.
South of Port Hope around Trapwood. There is also a group of 3 voodoomasters and 2 crypt shamblers on the shore east of Trapwood.
Knights: It would be wise to carry mana potions and plenty of food since you will be using exura to get rid of the paralyze and dworcs don't drop food. Close in fast and take them down.
Paladins: Should use any good ranged weapon to kill them without need to move, that way you wouldn't get hit by the poison storm and you would need a Dwarven Ring less, if you use a short range weapon you will have to run to them, so bringing a Dwarven Ring and extra health potions would be useful.