It only took a millisecond.

"What's that noise?"
"Don't worry, it's probably just a rat, the place is crawling with them"

The genestealer burst from the vent above, ripping the marine's helmet off in one sweeping motion.
Instinctively he caught the lashing front claw in his powerfist, and tried to get a couple of burst off with his bolter,
but the beast was too strong and pushed the weapon out of the way, the blasts harmlesly impacting the pipe behind.
Locked in a mortal embrace, they stumbled backwards into the gaping cavern below.

...

A small diorama of a duel between a Deathwing Terminator and a Genestealer onboard a Space Hulk.

The base is scratchbuilt from card, paper roll and bits and pieces ripped out of an old printer.
The head of the marine and the little rat are both sculpted from greenstuff.
The terminator and genestealer have been modified into their final positions with lots of patience and super glue.

The diorama is 80mm tall from the bottom of the base to the top of the vent pipe.

There are more photos here:
https://www.artstation.com/artwork/Oy9aNJ