Syndra Silvane described it as “a cup… the most bejewelled and decorated item in the entire lair, and under the most guard, right at the back.” It sat on red velvet in an iron-bound chest between two lightning pillars, radiating heavy necrotic magic (S1).
Quill put it, cushion and all, into his bag. Zaldara Kordris appeared and killed the party. It is presumably back in her hands.
Not to be confused with the runed “phylactery jar” in Syndra’s curio room, which is a Halloween prop.