eldritch EL-drich, adjective:
Strange; unearthly; weird; eerie.

Eldritch perhaps derives from a Middle English word meaning “fairyland,” from Middle English elf, “elf” (from Old English aelf) + riche, “kingdom” (from Old English rice).

antiphony (an-TIF-uh-nee) noun

Responsive alternation between two groups, especially between singers.

[From antiphon (a song sung in alternate parts), from Middle English, from Greek antiphona.]