@Daniel huh. I last played D&D version 3.5 and I don't recognize this (in this context), but they're up to v5 now. According to
the spec, the lich's phylactery holds its "life force" (so you can only destroy the lich by destroying this too), and it usually takes the form of a "sealed metal box containing strips of parchment on which magical phrases have been transcribed". Huh.