The cult of Mithras was a men-only cult based on Iranian deities that spread across the Roman empire and was especially popular with the army. Many forts in Britain had altars to Mithras the most northerly being Musselburgh/Inveresk fort as well as Mithraeums in London and St Albans.
Ceremonies were held in caves and other simple places. Benches up each side led to an altar and representation of Mithras with his attendants. Lighting sound and smells would enhance the whole experience.
At the top of the Mithraeum, there would be a relief of Mithras. The sun god Sol , the moon god Luna and two attendants, Cautes and Cautopates, with torches – one held aloft and one to the ground. A dog, a snake and a scorpion attack the bull from below.
Map of Mithras sites in Europe
Map above made by the people at https://www.mithraeum.eu/
Some useful links for further reading