In 'Malazan Book of the Fallen', Anomander Rake's sword that contains a dark realm is Dragnipur.
Dragnipur is a colossal, black sword forged by the elder god Draconus. Its most terrifying feature is that souls claimed by the blade are drawn into an internal realm, where they are forced to pull a massive wagon. The wagon is linked to the Gate of Kurald Galain, the realm of Darkness, while Chaos relentlessly approaches from behind.
Anomander Rake, Lord of the Tiste Andii and ruler of Moon's Spawn, eventually becomes the sword's most famous wielder. The weapon is far more than an exceptionally sharp blade: being killed by it can imprison a soul in Dragnipur's supernatural landscape rather than simply end a life. This makes it one of the most dangerous artifacts in Steven Erikson's setting.
A common mix-up is confusing Dragnipur with other named weapons in Malazan. Hust, Vengeance, and Grief are associated with different characters or weapons, but none is Rake's realm-containing sword. The sword and its internal realm receive especially important treatment in 'Toll the Hounds', the eighth main-series novel.