Marv, along with Rachel, Hank, Wilf and Sarah, may be encountered during Temple Trekking. The challenge involves quickly feeding them before they turn into ghasts. If this happens, the ghasts must be killed.
He can be fed almost any normal cooked food, but in case the player does not have any food, then the nearby snails can be killed and cooked for him to eat.
High level followers will help with this by starting a fire.
The longer he is left without food, the more he starts to resemble a ghast before finally turning into one.