Bone Spirit is the last of the three Bone spells available to the Necromancer. It provides more damage for a higher mana cost, and in addition it is also homing, meaning even if the enemy moves it will follow and hit its target, so long as no obstace comes in its way.
Bone Spirit starts out initially strong, but eventually fades in power compared to Bone Spear as synergies are invested in. Nevertheless, the homing effect can be extremely useful, especially in PvP.
Lore[]
This powerful spell briefly summons the spirit of a vengeful revenant. This skeletal specter immediately seeks out its objective, ripping free a portion of the target's soul and carrying it away to the plane of the dead. This is not a true summoning spell, however, as the Necromancers have not yet determined how to control these wraiths. For now, it is enough that they are able to shield themselves and their companions from their wrath![1]
References[]
↑Diablo II Manual, page 75. Blizzard Entertainment, 2000.