These sickening-looking trees grow only in the deepest caverns, and have the dubious distinction of being the most dense tree in Dwarf Fortress, with the Glumprong ranking in second place. Blood thorns are approximately half as dense as most stones.
They are also unique in that they produce crimson wood, which can be made into (heavy) crimson wood products, ideally ballista bolts (if you're not using metal arrowheads). Curiously, trampled young blood thorns are purple rather than dark gray, presumably due to the plant having been bruised.
Blood thorns are unique in that they can grow in cavern layers that are devoid of water - when this happens, blood thorns will be the only plants present.
[PLANT:BLOOD_THORN] [NAME:blood thorn][NAME_PLURAL:blood thorn][ADJ:blood thorn] [USE_MATERIAL_TEMPLATE:STRUCTURAL:STRUCTURAL_PLANT_TEMPLATE] [BASIC_MAT:LOCAL_PLANT_MAT:STRUCTURAL] [USE_MATERIAL_TEMPLATE:WOOD:WOOD_TEMPLATE] [STATE_NAME:ALL_SOLID:blood thorn] [STATE_ADJ:ALL_SOLID:blood thorn] [PREFIX:NONE] [STATE_COLOR:ALL_SOLID:CRIMSON] [DISPLAY_COLOR:4:0:0] [TREE:LOCAL_PLANT_MAT:WOOD] [TREE_TILE:181] [DEAD_TREE_TILE:181] [PREFSTRING:sickening appearance] [WET][DRY] [BIOME:SUBTERRANEAN_WATER] [BIOME:SUBTERRANEAN_CHASM] [UNDERGROUND_DEPTH:3:3] [SOLID_DENSITY:1250] [TREE_COLOR:4:0:0] [DEAD_TREE_COLOR:5:0:0] [SAPLING_COLOR:4:0:0] [DEAD_SAPLING_COLOR:5:0:0]