Acherus Deathcharger

An Acherus Deathcharger is the Dark Rider of Acherus's mount. Death knights must take the mount from the Dark Rider of Acherus while on the quest in order to learn how to summon Deathchargers of their own.

In Wrath of the Lich King
In phase two of the Scarlet Enclave instance, after completing, they can be found in the stalls in the Havenshire Stables and pastured in the adjacent fields.

The Acherus Deathchargers that are ridden by death knight players were once stolen Havenshire Stallions, Mares, and Colts, originally mounts of the Scarlet Crusade which were guarded by Stable Master Kitrik at the Havenshire Stables. One of these Havenshire horses captured and mounted by the death knight player in the quest and then returned to Salanar the Horseman. The Horseman instructs death knights in their capture, transformation, and acquisition by the death knight player as a Deathcharger mount of their own. The returned Havenshire steed is taken to the Shadow Realm and slain by the Dark Rider of Acherus, and then resurrected it in its presumably undead shadow form as an Acherus Deathcharger. The death knight player acquires this mount through the quest by slaying the Dark Rider and overtakes its mount in order to win back the transformed steed.