The Ghosts Warriors are two unaffiliated groups of antagonists from Ninjago: Masters of Spinjitzu introduced in season five, Possession. One group consisted of ghosts from the Cursed Realm in the employ of The Preeminent, but was later destroyed; the other previously consisted of Sensei Yang and his students, but has since been disbanded.
The Cursed Realm, as one of the Sixteen Realms that included Ninjago, was home to the spirits of beings who through various means had become cursed and thus ended up in the power of the Preeminent, who was both the queen and the physical embodiment of the realm. Desiring to spread her power across the realms, the Preeminent selected Morro, Master of Wind and former pupil of Sensei Wu, to seek out the Realm Crystal in the tomb of The First Spinjitzu Master.
When Sensei Yang used the power of the Yin Blade in an effort to make himself immortal, he instead cursed himself, his students, and the entire Temple of Airjitzu, leaving the building a dilapidated ruin and himself and his students as ghosts. Using his new dark powers, Yang enslaved his students to his will, and for years afterwards haunted the temple.