The day a dragon was killed by an illusion

It all started with an adventure though the dreaded Underdark. We ran into a lot of terrors down there but the worst was when we came across an ancient dwarven mine shaft. Which was a large circular tunnel about 5 miles deeper into the dark! Instead of using wth stairs down we decided to rig up poorly designed Parachutes and jump down. It worked for a while until we floated by an adult black dragon climbing up the shaft! Long story short the dragon chased us down the shaft as we plummet to the ground. The wizard gnome in the group used his fly spell to make it to the ground first. He then used hallucinatory terrain making the ground seem further away than it really was. Causing the dragon to smash into the earth killing it on impact!