A large group of units, generally with ranged attacks, where the most effective positioning is clumping them up into a ball. They hit a certain tipping point where fewer and fewer things work to counter them. If both sides have a death ball, then the smaller one loses by a bigger margin the larger the death balls get.
In a made up but typical RTS, one swordsman beats an archer, ten archers are an even match for ten swordsmen, and fifty archers beat infinity swordsmen. In a 10v8 archer battle, the bigger side survives with about 3 archers, but in a 100v80 archer battle, the bigger side survives with over 50. This game has archer death balls.
Most games will counter this effect with area of effect damage. AoE has onagers for this purpose - which can easily deal immense damage to clumped and unaware archers - SC2 actually gives a lot of units AoE which is what prevents marine deathballs from being nearly as strong as they might otherwise be. 10 zealots and a clossus vs. twenty marines will be a pretty one-sided fight that will get more extreme the higher the numbers go.
In a made up but typical RTS, one swordsman beats an archer, ten archers are an even match for ten swordsmen, and fifty archers beat infinity swordsmen. In a 10v8 archer battle, the bigger side survives with about 3 archers, but in a 100v80 archer battle, the bigger side survives with over 50. This game has archer death balls.