Underground Crimson spreads just like surface Crimson, and similarly can not spread to The Hallow (nor to Underground Hallow). It operates similarly to surface Hardmode Crimson, being comprised of Crimstone and Crimsand, though it spawns different enemies. It is the underground version of the surface-level Crimson, and appears at and below the cavern layer.
The Underground Crimson is a Hardmode biome, being one of the biomes that spawns when the Wall of Flesh is defeated.
