A consequence of bringing Buffy back has caused a demon to be created in Buffy's reality.  This demon is incorporeal but can take on forms and possess members of the group.  They determine that the demon will fade.  But they also find out that it can remain if it kills Buffy.  When they realize that they were overheard, Willow and Tara perform a spell to make is solid enough for Buffy to fight when it attacks.  After all is said and done, Buffy confides in Spike that her friends have taken her out of Heaven to return her to Earth.