A Necromancer is treated as a Zombie for all game purposes.
When a Necromancer appears, put an additional Zombie Spawn token (on its “Necromancer” side) on his entry Zone. Then, immediately resolve a regular Zombie Spawn for it. From now on, the Necromancer Spawn Zone is active: it spawns Zombies in the same way a standard Spawn Zone would. Each subsequent drawing of additional Necromancer Zombie cards, no matter the type, gives an Extra Activation to every Necromancer on the board until they flee or die. The game is immediately lost if a Necromancer manages to escape once there’s a total of 6 Zombie Spawn tokens on the board (including the Necromancer Zombie Spawn tokens).
A Necromancer tries to flee the board. If there are no Survivors in his Zone for him to Attack during his Activation (see Zombies' Phase, Zombicide: Black Plague Rulebook p. 23), he Moves 1 Zone toward the nearest Spawn Zone (NOT his entry Zone), ignoring any Survivors he might see. If several escape Spawn Zones are at the same distance, choose one.