Experience Level Advancement
Necromancers advance in levels like most specialists, as outlined in the PHB. While most player characters could spend their entire careers reaching 20th level, NPC wizards created by the DM can be far more powerful. Unlike many other character classes, the spell abilities of mages continue to progress beyond 20th level, and wizards continue to gain 1 hp per level advanced after 20th.
An expanded level advancement table is provided in Table 2 (on page 11). As with most wizards, necromancers with an intelligence of 16 or greater gain a 10% bonus on experience points, though this is normally not of much concern for the NPC, whose experience is designated by the DM.