The Nighthold is located in Suramar. This is the palace from which Grand Magistrix Elisande runs the city of Suramar. She has brokered a deal with theBurning Legion to allow them to use a portion of the Nightwell energy.

Dungeon Journal

The largest structure in the Broken Isles, and among the grandest in all of Azeroth, the Nighthold stands as a testament to the achievements of the nightborne civilization. Centered around the Nightwell, the fount of arcane energy that has sustained Suramar for centuries, these grounds were built as a haven from the worries of the world. But as a felstorm churns above the former temple of Elune across the bay, and Gul'dan himself now resides within the palace's walls, those worry now begin, not end, here.

Loot

The tier 19 set pieces drop from bosses in the Nighthold.

Encounters

• Skorpyron – Deep within the foundations of the Nighthold, beneath the sea, lie long-forgotten vaults that give access to the Nightwell itself. This monstrous armored scorpid has made its home in one of these vaults. Infused with the power of the Nightwell, and surrounded by a teeming brood, Skorpyron presents a serious complication to an otherwise-promising back entry to the Nighthold.
• Krosus – This colossal doom lord, one of the largest and mightiest creatures in the armies of the Legion, was defeated at the Broken Shore through the combined might of the greatest heroes of the Horde and the Alliance. Having recovered from his wounds, Krosus emerges from the bay between the Nighthold and the Tomb of Sargeras to crush any who would oppose the Legion.
• Chronomatic Anomaly – The cavern at the base of the Nightwell is a maelstrom of raw energy, as the power to fuel an entire civilization courses from the earth. Born from this chaotic flux, the Chronomatic Anomaly is an embodiment of the power of the Eye of Aman'thul. As it lashes out with energy attacks, the bursts of energy warp the very flow of time.
• Trilliax – Trilliax, once proud servant to the nightborne aristocracy, has been discarded and left to slowly deteriorate among other surplus constructs. While an unwavering will to carry out its tasks remains, the passage of time has splintered this construct's personality matrix. It now unpredictably switches from one mode to the next, ranging from doting caretaker to homicidal sterilizer, craving recognition and validation from a master that no longer exists.
• Star Augur Etraeus – The nightborne astromancer Etraeus has devoted long years of research to scouring the skies of Azeroth, seeking answers to the great mysteries of the universe. His scrying has shown him worlds beyond our ken, and the power of the Nightwell allows him to draw upon the essence of those worlds to amplify his own powers.
• Gul'dan – The chain of events set in motion when Garrosh escaped to Draenor has continued, unbroken for all its twists and turns, leading to this very moment. Thwarted in Draenor, Gul'dan now stands on the precipice of achieving ultimate victory on behalf of his Legion masters. Standing atop the Nighthold, as the vortex of fel energy swirls in the skies overhead, the outcast orc warlock awaits his destiny.
• Grand Magistrix Elisande – Elisande once distinguished herself by resisting the Legion. She and her highborne followers broke away from Queen Azshara and the dark path she was taking, harnessing the power of the Eye of Aman'thul to create the Nightwell to protect Suramar from the Sundering. But over ten thousand years later, the Legion's arrival offered her no such recourse. She has cast her lot with the demons, hoping to once again use the power of the Nightwell - albeit this time in a very different sense - to save her people.
• Spellblade Aluriel – Aluriel always had an affinity for magic. She rose through the ranks of the Nightguard effortlessly, having a natural talent with the sword. But no matter how strong she became, she wanted more. She studied with the mages at the University of Suramar, spending her days in combat training and her evenings in the artificery. She forged her weapons and armor in the Nightwell, weaving magic spells into the precious metals. She is the first Spellblade, adept in the schools of Fire, Frost and Arcane.
• High Botanist Tel'arn – From his youth, the nightborne Tel'arn was fascinated by plant life: the resilience and adaptability of weeds, the ability of simple grass to harness the power of the sun, the way a tree may be divided into two, or two branches grafted into one. Aided by the energies of the Nightwell, he has transformed himself to the point that he is scarely recognizable as a nightborne elf. He now considers himself something far, far greater.
• Tichondrius – The dreadlord Tichondrius, once leader of the nathrezim, was once slain by the newly-awakened power of none other than Illidan Stormrage. Reconstituted in the Twisting Nether, Tichondrius returns to watch over Gul'dan on behalf of the Legion, ensuring that the orc warlock does not once again fail his masters.