Demon Army was a challenge introduced in Series 7, utilising a catapult to shoot at Demons.
Used on Monday or Tuesday of a heat or final week, the six warriors are split into two teams of three, and set against five Demons, three in front and two behind. Four of the Demons are the 'everyday' Demon variety, dressed in the uniform brown cowl, but the other is a senior Black Demon (who takes centre stage). More on him later.
Anyway, the three warriors had to use a giant catapult (two holding the ends, the other firing it), and shoot rocks at the Demons, who'd gradually step forwards (one step at a time), edging towards a line drawn on the ground in front of the line the warriors had to stay behind. If a rock hit a Demon, he'd be destroyed.
However, if the warriors shot the Black Demon, he'd disappear in a white flash like his subordinates, but he'd rematerialize further back, either level with the rear Demons or behind them. If the warriors wanted him 'properly' dead, they'd have to shoot him twice, as he could only rise up again once.
The aim was to kill all 5 Demons before the came beyond the skulls. If any/all of them did, the screen would turn transparent orange and the warriors would vanish.
There were three rings at the line the Demons were walking to, so the warriors would get one ring apiece. Later seasons would introduce the art of Demons leaving rings at the point they were shot at, which sometimes led to unequal numbers of rings for the warriors (if there were three rings on the skull line).
The team facing this challenge had to operate a catapult which fired boulders at encroaching Demons. The Demons would take steps forward after every shot and the team would lose the challenge if a Demon reached the line. The Demons were arranged in a triangle so the centre demon would reach the line first, followed by the next two back, and then the final two. The warriors have at least fifteen boulders to complete the challenge with.