It is an expansion for Santorini (2nd edition).
Santorini: Hiding in the Underworld contains 17 characters:
- Arachne
- Cerberus
- Charon
- Chronus
- Hades
- Hecate
- Hypnus
- Mnemosyne
- Moerae
- Morpheus
- Nemesis
- Orpheus & Eurydice
- Persephone
- Phobos
- Sisyphus
- Tartarus
- Typhon
It also contains 2 mode cards:
- Nyx's Veil
- Manticore's Shadow