The Raider is adorned with skeletons and skulls impaled on staves, while prisoners taken in battle are tied to its decks with barbed filaments. The enemy troops find this immensely disturbing and threatening, so any enemy unit with a model within 6" of the Raider must subtract -1 from its Leadership value. Note that a unit suffers a maximum penalty of -1 to its Leadership regardless of the number of Raiders with Trophy Racks within 6".
Trophy Racks
Source: Codex: Dark Eldar (2002), p. 13