The Shadow of the Sentinax

Killing mobs under the Sentinax ship rewards [item=147356], Which starts the quest The Shadow of the Sentinax Turn this in to [npc=120414] at Deliverance Point. You'll then be introduced to the various items you can buy with Nethershards.

Related Items
These beacons will create portals that summon progressively more-challenging demons:

Uncommon Beacons:

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• [item=146909]
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• [item=146908]
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• [item=146907]
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• [item=146906]
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• [item=146905]
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• [item=146903]
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Rare Beacons:

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• [item=146915]
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• [item=146914]
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• [item=146913]
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• [item=146912]
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• [item=146911]
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• [item=146910]
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Epic Beacons:

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• [item=146921]:[npc=120686]
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• [item=146920]:[npc=120681]
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• [item=146919]:[npc=120675]
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• [item=146918]:[npc=120665]
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• [item=146917]:[npc=120641]
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• [item=146916]:[npc=120583]
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There are also some beacons with profession benefits:

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• [item=147355]: Commands the Sentinax to fire a Bloodstrike at your current location, unearthing Blood of Sargeras. Deals 150% of maximum health as Chaos damage.
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• [item=146923]: Commands the Sentinax to fire a Petrum beam at your current location, turning everything in the area to felslate.
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• [item=146922]: Commands the Sentinax to fire a Fel Seed at your current location, causing starlight roses to rapidly grow.
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The wording of this comment is misleading: To be clear, you cannot buy Beacons with Nethershards. Also, you are able to buy the things this NPC sells without ever looting the Broken Beacon or turning in the related quest. Shame, really... would've loved to be able to spend Nethershards on Beacons.