It's a common term in modern RTS, and it's got more meaning attached to it than simply "a large group". For example, in StarCraft 2, a full army of zerglings wouldn't be considered a "death ball" because it'd be so easily defeated with a few Colossus and high templars (units that deal a lot of splash damage, which zerglings are particularly susceptible to). A death ball is an army so powerful it's virtually unstoppable once it's formed. Players can try to achieve a death ball to gain a victory, but it's often expensive and leaves you vulnerable while you're building one. Also death balls are usually slow, and the opponent can use faster units to attack vulnerable parts of their base while the player building a death ball is out of position.
