Overview

This build is effective, with inexpensive gear options while remaining viable. He is a powerful early ladder option where you can’t rely on accumulated wealth and stashed D2 Resurrected Items. Likewise he can be viable in Hardcore with the right gear selection. The varied damage sources mean that he won’t need to use Sunder Charms to remain effective and won’t need to offset negative resistances. As a PvM Character, your effective clear speed will be on par with higher cost builds, and will require significantly less optimization to remain effective (due to Corpse Explosion damage scaling independent of skill or gear investment).
The emphasis of this build is on life, mobility, powerful bone attacks that are rarely negated, corpse explosion, curses to amplify damage, and summons to act as a shield and provide additional physical damage.

Attributes
Enough strength and dexterity for your selected gear, the rest in vitality.

Skills
You have some autonomy here, depending on your preferred playstyle. A summon primary build will afford more tankiness to minions and thus more safety. However, it will lack the initial clear speed of a bone primary build. Summon primary is recommended for Hardcore.

Bone Tree
There are three primary bone attacks you can choose from. Teeth, Bone Spear and Bone Spirit. If you are Bone Primary, it is recommended you invest 40-60 points in this tree. If you are Summon primary, max one bone spell and invest one point in necessary prerequisites. Invest 1 in all other synergies (Bone Wall and Bone Prison).

Curse Tree
One point each in your desired curse(s). Amplify damage requires the least skill investment (a single point) and provides ample damage amplification for minions, corpse explosion and your mercenary. Decrepify and Dim Vision can provide additional safety for Hardcore. 

Summon Tree
Your remaining Skill points will be used on Summons. A Golemancer build may be viable but is not recommended as a single Golem will not be as effective as dispersing damage. An Iron Golem will however, be utilized for an aura. 1 Point in Revive, Skeletal Mages, Golem Mastery, each Golem, and Summon Resistances. Next max Skeletal Mastery, then Skeletal Warriors.

Iron Golem Aura
Insight - This will be a cheap option which should alleviate the need to use mana potions.
Pride - This can be considered for a high cost build, and where you are not using an A2 Pride mercenary. 

Bramble - Another viable option providing thorns aura to your army when you are not using this on an A2 mercenary. 

Gear

Helm:
Shako - Best all round caster helm where FCR is not required
Crown of Ages - Best defensive option for Hardcore

Amulet:
Maras - Best for softcore and Hardcore (FCR is not required for this build)

Weapon:
Heart of the Oak - Best all round caster weapon for Bone Primary build
BeastZ - Best minion buffer for Summon Primary build
Bone Wand - A strong offensive orientated option for Bone Primary build

Rings:
Bul Kathos is recommended. Stone of Jordan is viable.

Shield:
Spirit Monarch - The best standard caster option if not opting for max block

Homunculus - A strong max block option
Darkforce - A good all round option
Stormshield - The best defensive option for Hardcore (note: you cap out at 50% damage reduction, which is something to keep in mind if also utilizing Crown of Ages).

Armor:
Enigma - The strongest option for any game mode, offering teleport repositioning for your army, as well as arguably the strongest general buffs from armor options. 

Vipermagi or other + skill armours which offer defensive stats can be considered budget options.

Gloves:
Trang gloves - These offer useful cold resistances, and a curse buff, as well as FCR. While FCR is not crucial to this build, it can offer some additional speed when teleporting.
Bloodfist - A great defensive option offering FHR and life.

Boots: 

Sandstorm Treks / Marrowwalk / Waterwalks / Shadowdancer (maxblock) / Rare Tri-res are all viable options depending on your needs.

Switch:
Call to Arms / Spirit Monarch standard switch.

Charms:
Annihilus / Torch / Gheeds (optional) / Sunders (not recommended)
Poison and Bone Skill GCs (Life Desirable) - Bone Primary Build
Summoning Skill Gcs (Life Desirable) - Summon Primary Build
Life / Resistances / Faster Hit Recovery, Small Charms

Mercenary:
Note, you should avoid freezing abilities on the Mercenary like Ice Arrow or Holy Freeze given they shatter corpses and prevent them from being used for Corpse Explosion. The Chilling Armor effect of Fortitude is negligible and does not prevent it from being a viable option. Additionally, if you are utilizing an Iron Golem for aura, and/or self-wielding aura items like Beast, then remember that the same auras do not stack (I.e. Faith and Beast)

Option 1: A1 (Fire Arrow)
Weapon: Faith GMB (The +skills you can roll on GMB’s now affect Mercenaries in D2R)
Armor: Ethereal Fortitude / Bramble
Helmet: Ethereal Andariel’s / Vampire Gaze

Option 2: A2 (Might / Defiance / Prayer)
Weapon: Pride / Infinity (for enhanced CE damage) / Insight 

Armor: Ethereal Fortitude / Bramble
Helmet: Ethereal Andariel’s / Vampire Gaze

Skill Rotation:
Ensure Bone Armor and Warcry effects are active at all times, including on your army. Curse, and kill the first enemies with your Bone Spells. A single kill in a packed group should allow you to chain CE the remaining packs, as your army provides an alternate damage source. Use Revives on larger front line minions.

Conclusion
The variability afforded by this build is due to your primary damage dealer (CE) requiring no real skill investment. As such, customizing your build will not hurt your efficiency as is the case with almost every other D2R build. Playing around with different options, or carrying switch gear is recommended to find your own sweet spot for your preferred playstyle. You will not be punished for not having expensive gear, though teleport lost from not equipping Enigma can be considered a major drawback for mobility and clearspeed if a teleport charge item is not used instead.

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Article From: D2R Hybrid Bone Summon Necro