The Warlock is launching across the entire Diablo franchise.
Blizzard is making an effort to unify lore across titles.
Our Heros will bind Bartuc in Act VI. 30 years later Bartuc returns as the main villain in Blizzard's flagship title, Diablo IV.
Our D2R Warlock is a quick re-skin of the necro à la Archivist April Fools.
https://dev.diablowiki.net/The_Archivist_(april_fools)
This deception allows Blizzard to playtest and balance the new skills for our existing characters prior to releasing Act VI.
Diablo II and Diablo IV Warlocks are from different Mage Clans. Observe stylistic differences:

The Warlock Class Cinematic Trailer depicts a Zann Esu mage releasing Bartuc in Diablo IV.
This rug pull will generate gravity and excitement for Blizzard’s priority: Diablo IV. Skip to 38:22 of the Diablo 30th Anniversary Spotlight. The employees are giddy.
Blizzard is orchestrating a multi-title BOMBSHELL.