Dark titans are a subrace of evil titans. The only named and known one is Sargeras.
Background[]
The only known dark titan is Sargeras, the Burning Legion creator. Sources say that many other dark titans exist but have remained unseen when trying to influence Azeroth. These titans may be the ones Sargeras corrupted and brought over to his side or perhaps rogue titans who have similar feelings to Sargeras. This is current unconfirmed, but it is possible Sargeras has many dark titan allies.