What is the meaning in a dream of cistern

I was in a police bureau with a policeman as bodyguard, then some 'demons' (weird masked mans) came to me via the roof, i was scared and i wanted to run away but the police man said:'sorry you cant leave this building now'. so jumped into a cistern to hide, the police man warned the demons with: 'don't come any closer or i will shoot!' but the demons didn't listen and they walked towards me, (knowing that they saw me there) i runned to a pillar and hided behind it, the policeman was in panic and gave me a gun (he still had his own), the demons broke the windows with explosions. in an in a fraction of a second: there were a lot of explosions and flying bullets, the agent shot a few times and died, then i started to shoot at them but they killed me before i pulled the trigger. i woke up with sweat all over my body

I had a dream about there’s a grave site with sealed containers in a metal cistern