This small faction was based in a outpost/trading post that saw a good amount of action in Episode 1 and 6. It was notably Anti-Trade Union. Edit
Tunnelside is named after the nearby road tunnel. It was established by the New Novgorod Republic as a trading outpost. It was situated on a hill and its small entrance provided a good defence. It's first major event was when a Trade Union convoy stopped at the tunnel and was fighting a three-way battle between a New Order patrol and some Cultists. Fearing the village would be cut-off from supplies, the outpost launched an attack on the remaining TU soldiers. It won the battle.
A few months later, the TU and NNR unified to create the Commonwealth of Novgorod. This, along with the embarrassing defeat at Veliky Novgorod, tempted many outposts and towns to break away. Tunnelside was one of these, also by far the strongest. They began their own faction, also known as the Novgorod Seperatists. They never stopped the war with The New Order, however. They quickly besieged the soon-to-be-evacuated base of Komosolkin.
Unfortunately for them, the Commonwealth did not forget about them. A large force was sent in to capture the settlement. By routing besieging forces at Komosolkin, the Commonwealth innavertedly allowed New Order troops to escape. Although the Seperatists inflicted heavy casualties and halted the first wave, the Commonwealth managed to penetrate inside the town and kill all defenders. In what is still a highly debated topic in Commonwealth households, the victorious army then dynamited the settlement. Tunnelside and the Novgorod Seperatists were erased from history.