Let me try to write a quick script.
The camera glides over a fortress built from marble, the fortress is built inside of a mountain. The camera moves in slowly.
The camera is moving through narrow, twisty halls adorned with masterful engravings.
The camera slows by some of the engravings, some of the engravings are of heroes, random things, and horrid forgotten beasts
the camera stops sideways near Urist McMayor and a uppity elf discussing their trade agreements. The room is smoothed out, and also made of marble.
Urist McMayor: I'm glad you are enjoying my fine mountainhome, now on to the agreement.
The Elf: True, it is a mildly good hole you've dug yourselves here. Anyways, we are lacking armor of all kinds, the war we are waging with the humans has gone on for too long. We also need prepared food for the armies as well, can you fulfill these is due time?
Urist McMayor: I have legions cooks beneath me as we speak and I have enough iron to fit a elephant for battle. So yes, I can fill these requests in due time, Elf.
The Elf: Very well! Very well! Your other dwarves request me for trading. See you next year fine dwarf.
The Elf stands, turns and walks, Urist McMayor looks at him, chin on fist, with a sour look on his face.
Loud metallic clinging is heard down the hall, alerting Urist McMayor.
Several dwarves clad in steel armor are barging down the hall.
Urist McMayor grabs a axedwarf by the arm, he begins to talk urgently.
Urist McMayor: What in the blazes are you soldiers running too!
The Axedwarf: I barely have time to explain, one of the marksdwarfs patrolling the depths saw a monster.
Urist McMayor: A monster?! The mad engraver's dream was true! The forgotten beast of the depths has resurfaced!
Several screams accompanied with the sounds of raging fires run through the halls.
The Axedwarf breaks the grip of Urist McMayor. Urist McMayor gives chase.
Urist McDwarf stares mouth agape at the monstrosity.
The forgotten beast is blows fire into the shields of several soldiers.
The Axedwarf: Rot in the underworld you beast! He charges.
The beast smashes The Axedwarf with his tail, sending him into a wall, he loses consciousness from the pain.
The beast is winning, the ground is littered with various corpses and wreckage of destroyed workshops.
The camera is following someones feet.
Urist McMayor: You...you fool, don't go you fool! Urist McMayor starts to sprint.
Time is slowed down, things are shown in extra gory detail.
The feet of the unknown sprinter are shown jumping on a damaged table, the sprinter does a high jump.
The sun is shown between the sprinter and the forgotten beast preparing to shoot another blast of flame.
The sprinter slams his fist into the beast's head, it collapses dead.
The sun lowers down more, showing the sprinter to be a mere child.
Urist McMayor collapses in front of the child.
Urist McMayor: ...s-son...
The son rushes to his father, Urist McMayor.
The Axedwarf and several wounded soldiers rise.
Urist McMayor's son embraces him.
The risen soldiers bow down before son, chanting his name.
THE END
Not exactly my best work, but then again, this is only meant to give a good idea of what I imagine a DF trailer would look like. I Could also do the situation about the adventurer killing a bronze colossus with fluffy wamblers.