The onrushing hordes of a Tyranid swarm are a terrible sight to behold: scuttling Termagants rub shoulders with deadly Genestealers, huge Carnifexes lumber forward beside tall Tyranid Warriors in an avalanche of rattling armor plates, glittering fangs and gigantic claws. Discipline is hard to maintain against such a horrifying foe as many men are driven mad with despair or frozen with terror at their approach.

- Inquisitor Agmar: Halting the Abomination

The Tyranids are an utterly alien force that is terror incarnate. Shapes of nightmare leap and scuttle forward with outstretched claws and glistning teeth pull down their prey. They are an emotionless force of genetically designed killing machines driven by the implacable will of the Tyranid Hive Mind. This Hive Mind is a type of collective consciousness that all the organisms of the Tyranids are a part of. They feel each others thoughts and it's this oneness that drives them on with such unstoppable purpose.

The armies of these aliens are exemplified by swarms of lesser creatures that need the guidance of larger bio-organisms that are more closely linked to the Hive Mind. Without this guidance these creatures soon become confused and lost in what it is they must do, causing them to revert to instinctive impulses, instead of that which the Hive Mind has planned. Through careful extermination of these leader creatures who act as "nodes" for the Hive Mind, a Tyranid force may be stopped, but this is easier said than done!