The Army of Darkness (called in the 4Kids dub as the Army of Decay), also known as the Dark Army and Creatures of the Dark, is a monstrous army made purely of dark magic and an Antagonistic faction in the Winx Club series. They first appeared at the end of the 1st season of Winx Club.
The Trix seemingly stole the Dragon Flame from Bloom and used it to summon the Army and destroy Red Fountain. When the Trix were defeated, the entire Army vanished before everyone's eyes because Icy, Darcy, and Stormy no longer had the strength to command it and because the power of the Army depended on that of the Trix.
One important characteristic is that for each monster you cut or kill, there will appear two more. The monster also can unite into one beast. Only a possessor of the Dragon Flame (or something with equal power) can summon it with its spell, though Miss Griffin told the Trix that nothing can control it when they revealed their intentions of summoning it.
However, since at the end of the season it was revealed that the Trix never really had the Dragon Flame, it remains unknown how they were able to control it.
It may have been that because the Trix thought they had the Dragon Flame, they gained the power to control the army.