In Diablo 4, the Necromancer class offers incredible damage potential, and in this guide, we'll focus on a powerful Necromancer build centered around the devastating Bone Spear ability. This Diablo 4 build is capable of dealing massive damage to both regular mobs and bosses, making it a formidable choice for endgame content. We'll break down the key elements of this build and explain how it works, as well as discuss the skills, passives, and legendary aspects that synergize well with it.

Build Overview:
The core of this Necromancer build revolves around maximizing the damage output of Bone Spear, which serves as the primary source of damage. Bone Spear excels in various areas, including high single-target damage, area-of-effect (AoE) damage, and piercing through minions. To enhance the effectiveness of Bone Spear, we'll focus on increasing its damage and exploiting vulnerabilities.

Skills:
Bone Spear: This is the primary ability that forms the backbone of the build. Upgrade Bone Spear's critical strike chance and increase the number of bone shards released on critical strikes to maximize its damage potential. The first enemy hit will apply vulnerability, synergizing well with the Splintering Spikes aspect.

Corpse Explosion: Corpse Explosion complements the build by increasing overall damage. Consuming corpses not only deals increased damage but also generates Essence, allowing for more bone shards to be cast.

Bone Storm: While not crucial to the build, Bone Storm provides additional critical strike chance while active, boosting damage output. The barrier gained from using Bone Storm can also provide some survivability.

Decrepify: The chosen curse, Decrepify, slows enemies and reduces their damage, aiding in kiting and improving survivability.

Bone Tendrils: Bone Tendrils serves as an effective AoE crowd control skill, pulling enemies towards you and stunning them. It synergizes with Halcyon Aspects' corpse explosions and applies vulnerability, amplifying damage.

Passives:
Memento: Sacrificing Skeleton Warriors and Skeleton Mages increases their sacrifice bonus by 60%. This bonus enhances Critical Strike Chance and Critical Strike Damage, both of which play a significant role in the build's damage output.

Essence Harvest: This passive amplifies damage for each point of Essence available, making Essence generation crucial. Corpse Explosion's Essence generation helps fuel the build's damage potential.

Bone Shroud: Bone skills deal increased damage to vulnerable enemies, which ties in well with Bone Spear's vulnerability aspect. This passive synergizes with the build's focus on vulnerability and amplifies damage against affected enemies.

Legendary Aspects:
Deathless Visage (Helmet): This helmet significantly boosts damage by leaving behind additional explosive orbs that provide AoE damage. It also offers excellent stats such as Physical Damage, Critical Strike Damage, Maximum Essence, and Total Armor.

Hands of the Howling Rift (Gloves): These gloves grant bonuses to Corpse Explosion, enabling summoned corpses to charge at enemies and deal bonus damage. While Mist Aspect could be an alternative, it may hinder visibility in the game.

Disobedience (Pants): Disobedience provides increased armor and is preferable for damage reduction compared to pants with barrier-related effects. Seek pants with damage reduction and additional defensive stats.

Conclusion:
The Necromancer's Bone Spear build in Diablo 4 showcases immense damage potential and is ideal for both endgame and boss encounters. By focusing on maximizing Bone Spear's damage output and exploiting vulnerabilities, you can obliterate enemies quickly. Remember to adjust and optimize the build further by acquiring better items, refining skill choices, and experimenting with other legendary aspects.