https://classic.battle.net/war3/undead/units/deathknight.shtml
Death Knights were once virtuous defenders of Humanity. However, once the Paladin ranks were disbanded by the failing Alliance, many of these holy warriors traveled to the quarantined lands to ease the suffering of those left within the plague-ridden colonies. Though the Paladins were immune to disease of any kind, they were persecuted by the general populace who believed that they had been infected by the foul plague. A small band of Paladins, embittered by society’s cruelty, traveled north to find the plague’s source. These renegade Paladins succumbed to bitter hatred over the course of their grueling quest. When they finally reached Ner’zhul’s icy fortress in Northrend they had become dark and brooding. The Lich King offered them untold power in exchange for their services and loyalty. The weary, vengeful warriors accepted his dark pact, and although they retained their humanity, their twisted souls were bound to his evil will for all time. Bestowed with black, vampiric Runeblades and shadowy steeds, Death Knights serve as the Scourge’s mightiest generals.
https://classic.battle.net/war3/undead/units/lich.shtml
During his mortal life as the Warchief of the Orcish Horde of Draenor; Ner’zhul commanded a number of Orcish Warlocks and Shamans. Yet, when these wicked sorcerers were captured by Kil’jaeden and the Legion after the destruction of Draenor, they were transformed into twisted aberrations of their former selves. These Liches possessed tremendous magical powers, yet their immortal, undead bodies were bound to the iron will of Ner’zhul. Since the Liches showed unswerving loyalty to their master, Ner’zhul granted them control over the furious elements of the cold north. Now, Liches wield frost magic along with their own considerable necromantic spells.
https://classic.battle.net/war3/undead/units/cryptlord.shtml
These ancient, evil behemoths were once the mighty kings of the subterranean spider kingdom of Azjol-Nerub. Though the Nerubians fell before the wrath of the Lich King during the fabled War of the Spider, the insidious Crypt Lords were swayed over to the Lich King’s ranks and granted considerable power and the immortality of undeath. Now the giant, thundering beasts serve as the Lich King’s mightiest warriors and the guardians of the Scourge’s holdings in Northrend.
My mind is made up by the Warcraft 3 description that explicitly names two of the Scourge’s three “undead” heroes as undead while the third, the death knight, is said to have retained their humanity even though their will is bound to the Lich King through their vampiric runeblades. Both necromancers and acolytes are also described as being alive despite being undead in terms of game mechanics.
https://classic.battle.net/war3/undead/units/necromancer.shtml
Though they retained their Humanity after making a pact with death, Necromancers became the most terrifying agents of the Scourge. These dark, insidious men were once thought to be aspiring geniuses by the Magocracy of Dalaran. However, their insatiable lust to delve into the secrets of the dark arts drove them to forsake their very souls. Ner’zhul, the Lich King, granted these malevolent sorcerers true power over the dead in exchange for their loyalty and obedience. The shadowy Necromancers have the power to command various types of Undead and even spread the foul undead plague as well.
https://classic.battle.net/war3/undead/units/acolyte.shtml
Acolytes are human beings who have given themselves over to the power of Ner’zhul and the Scourge. These bitter, fanatical men and women will stop at nothing to promote the Lich King’s will and maintain the secrecy and dominance of the Cult of the Damned. They view their own deaths and the possibility of becoming undead as the ultimate rewards for their service to Ner’zhul.