I'm in the Cenotaph Vault. Put the dead rat on the alter and evoked The Bane of Hobnob from my inventory, but nothing happens. How do I open the door with the skull on it to get into the rest of the vault ?

GOG game forums can be dead unfortunately (especially if a game doesn't sell many copies), so I'm skeptical you'll receive a response here, but you could try asking on the General Discussion forum or messaging the developer or asking on the Steam forum for this game