The Siegebreaker snatch is a scripted event in which the unseen demon reaches through a wall and snatches a hapless NPC to his doom. It's seen in the WWI 2008 gameplay movie at the 10:00 mark.
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The Barbarian runs in from the right, and sees the NPC standing to the left.
With a great rumbling and shaking, the wall crumbles as a huge hand appears.
The hand closes on the NPC and yanks back out of sight.
Only rubble remains from the collapsed wall, while one of the other NPCs reacts in shock.