Complex sentence: Within six months, a 70 people detachment that was left in the Nerubayske catacombs were active in sabotage and intelligence work, before they were immured by Romanian invaders and almost all of them died.
Split into simpler sentences: Within six months, a detachment of seventy people that was left in the Nerubayske catacombs became active in sabotage and intelligence work. They continued until they were immured by Romanian invaders and nearly all of them died.