Necromancer was a respected professor but when he started running tests with life and death, he was asked to leave his academy. However, Count Vlad offered his castle to Necromancer: He could carry out his experiments there and the Count could show off his influence: he had a Warmaster living in his home.
"Klaatu Barata Nikto" is a famous phrase from the film The Day the Earth Stood Still. The phrase would be said to a robot to not destroy the planet and cause it to resurrect the alien, Klaatu. The phrase is used in the film Army of Darkness where Ash mispronounces the phrase as "Klaatu Verata Nectar".
Necrolicht was Necromancer's brother, imprisoned in Hell by Barbatos out of revenge for all the souls that Necromancer had wrested back from the afterlife. The years it took for Necromancer to reach and free him took a toll on his spirit.
Warmaster Sherezar, Necromancer's friend, sacrificed himself in battle against the Titans. Necromancer resurrected him.
While scouting a ruin as a location to found a school of dark magic, Necromancer used a magic lamp to summon the genie Miserus, but came to regret it as the genie was more powerful than him in death magics. When he threw the lamp into the sea, a second genie, Abysmuss, simply arose.
Lord Pumpseed was created when Hayman accidentally ruined Necromancer's plan to resurrect and control the monster, leaving Pumpseed to roam free.