The First

Claiming to be the supreme evil, the First appears in season three episode 'Amends' when it mentally tortures Angel, desperate to turn him bad again. Using his weaknesses and the guilt of his past crimes, the First takes the appearances of his victims. As it morphs, it tries to convince him to join it, that killing Buffy will make his suffering stop. Angel can't and would rather take his own life.

Once Spike gets his soul back, the First appears once again and tries to turn him bad too. Spike doesn't give in either and keeps his faith in Buffy. The First also finds a way to revert Spike into a brutal killer without him knowing. As if he was hypnotised, a simple trigger sets him off. Spike fights this and begs Buffy to help him.
It also used Andrew, leading him to an amulet and pushing him to kill Jonathan.

When the First attacks someone, it seems to drive the person insane, or at least extremely confused. Angel was very disturbed and Spike goes around speaking nonsense.

Not actually a being, The First cannot act itself, it must control others and it's not technically killable. It has the power to morph in anyone to confuse its victim. It also seems to enter and manipulate dreams.

The Bringers, sort of evil high priests, appear with the First. It's hard to say what they are exactly, but their eyes are not visible, replaced by cross-shaped scars.

In 'Amends', Buffy locates the First, thanks to Giles indication that nothing can live below or under it. Several years later, it's still in the same place, allowing Buffy to find it once more.