Bone Prison is similar to Bone Wall in that it prevents monsters from reaching the Necromancer, keeping him at a safe distance. Their hitpoints are almost identical, although Bone Wall has slightly better synergy bonuses.
Similar to the Bone Wall, a Necromancer with this skill can summon a spirit wall of bone in the form of an enclosing ring. This prison traps all enemies within its circumference. Imprisoned within a cage of bone, the captives are held helpless until they can summon the strength to break through its skeletal confines or the Necromancer loses his hold over the spirits.
↑Diablo II Manual, page 74. Blizzard Entertainment, 2000.