You suspected correctly.
Agatha Harkness! She is not a villainness in the comics, but a wise old witch who helped Wanda learn to use her powers more effectively.
But that book that was emphatically shown might be the Darkhold, so maybe she is in turn being manipulated by the sinister Cthon.
(In the comics universe, the Earth was initially inhabited by the Elder Gods, immense, etherial life forms who fell to war and strife, devouring each other; Cthon is one of the most cunning and cruel and one of the few to survive, by taking shelter in another dimension. In order to facilitate his eventual return to Earth he left behind a batch of indestructible manuscripts of magic spells, which would draw evil people to them, hungry for the power they promised and would intentionally or unintentionally open the door for his return. Those scrolls were eventually bound into a tome known as the Darkhold. Wanda has a mystical connection to Cthon because her powers come from the same source as his, If I remember Correctly.)