Note: This minicryptic was written on the occasion of the series finale of Buffy the Vampire Slayer. While it does not contain spoilers for the series finale, it does make reference to various events of the preceding seven seasons, so those who are behind in their viewing and are scrupulously avoiding spoilers should probably duck out now.
This crossword contains a magic talisman to protect the three classes of grid entries: (1) those that do not overlap it at all, (2) those that overlap it in part, and (3) those that fall entirely within it. Read the first letters of the clues for class 1, then the second letters of the clues for class 2, and finally the third letters of the clues for class 3, to get a famous tribute to the magical entity also described in the title.
ACROSS
1. Still-beating heart, perhaps from St. Paul or Mobile (8)
6. The mummy Xander dated, or anyone like her in California (4)
7. Grabbing back what is held by insecure Potentials (4)
8. Demonic stake is in toolbox when Angel finishes (6)
12. Solo's right for Anya's first musical performance (5)
13. Two in Cordelia's circle talk about site near pyramidal tombs (5)
15. Shapechanging insect is most agreeable (6)
16. Herb for a Wise One (4)
17. Eldest of three winged imps is colorful? (4)
18. Say, Willow describing a small Sumerian city's riches (8)
DOWN
1. A scaly creature William the Bloody decapitated (4)
2. Vain, vile being inside Sunnydale or Berkeley computer (6)
3. So after that Oz really is chiefly in a Zen state (6)
4. A worldwide computer system expels Moloch finally, it is believed (5)
5. Act like a corpse on a roll (4)
9. Primarily, Scott Hope flip-flops about 100 times (6)
10. Eliminating device seer fashioned to imprison
vampire-slaying deity? (6)
11. Eat like crazy twisty plant that's swallowing Giles's head (5)
13. Demoniacal acts and actors (4)
14. A departing Bone Eater's left eye's swelling (4)