Necrotic Orb is a skill located in the Undaunted (which can be found in the Guild skill tree). Necrotic Orb is a base skill, and can be morphed into Mystic Orb or Energy Orb.
Necrotic Orb deals the following types of damage: Magic Damage. Which means that it gets buffed by the following blue champion point trees: Elemental Expert, Elf Born, Spell Erosion. Also the following red champion point trees protect against this skill: Elemental Defender.