The heart of the Crystal, wherein lies the soul of the star and it is in this place that the King will gain the power to fulfill his calling.Bahamut describing his realm.
The beyond or the heart of the Crystal is a realm between life and death in the Final Fantasy XV Universe. It appears in Final Fantasy XV, Final Fantasy XV: Episode Ardyn and in Final Fantasy XV -The Dawn of the Future-, and is the domain of the Astral Bahamut.
The realm is not directly named, but Bahamut calls it "the heart of the Crystal", whereas Ardyn Izunia refers to his passage there as "I will await you in the beyond".
The realm appears as a borderless blue void bathed in soft light and appears connected to Eos's Crystal. Those who leave this realm, seem to manifest at Angelgard. In The Dawn of the Future, the realm is described as a "space connected by the Crystal, where none can cross over in the flesh."
The background of the bestiary and dossiers entries is the beyond. Crystals being manifestations of crystallized consciousness in some manner is a Final Fantasy series trope, and in past Final Fantasy games people's souls have returned to the planet's crystal, but the nature of the afterlife in Final Fantasy XV is ambiguous. The beyond is somewhat akin to the unseen realm in the Fabula Nova Crystallis: Final Fantasy, the afterlife dimension of the universe where nothing has form and where the fal'Cie believe their gods to reside.
Now it is over, Majesty. What will you do? Banish the daemons and bring peace? Erase me from history once more? ...I will await you in the beyond.Ardyn after being defeated by Noctis
Passage to Bahamut's realm is granted via different means. In Final Fantasy XV: Episode Ardyn, after Bahamut stops Ardyn from killing King Regis, he brings Ardyn to his realm to reveal the latter's true calling: to spread the Starscourge around Eos and then be killed by the True King, granting him both his revenge and eternal peace. Ardyn can either accept or rebel, but is ejected from the realm and finds himself in his old cell in Angelgard.
In Final Fantasy XV Noctis enters the realm directly by touching the Crystal and being absorbed by it. He meets Bahamut who tells him the Crystal holds the "soul of the star" and explains Noctis's destiny as the True King: as the culmination of the Lucian kings' bloodline, Noctis can save Eos from the Starscourge—a malady that engulfs the world in darkness and turns humans into daemons—by defeating Ardyn, Noctis's immortal ancestor who once fought the Starscourge whose own soul became corrupted by it. Noctis sleeps in the heart of the Crystal for ten years while his Ring of the Lucii absorbs the stone's power. After Noctis entered the glimmering Crystal, its light faded and it became lusterless.
Final Fantasy XV -The Dawn of the Future- depicts Noctis entering a dream world and receiving various memories from the Crystal during his decade-long sleep in its heart, coming to understand the history of the world and his lineage, and the motives of his nemesis and beloved. The spirit of Noctis's father, King Regis, appears to him in his dream in the beyond to offer encouragement.
Noctis fulfills his calling and opens the way to the beyond with the power of the Ring of the Lucii, dying in the process. He meets Ardyn's spirit in an in-between realm that appears as a glimmering void identical to the place where he had met Bahamut. In the original game whether the two are one and the same is ambiguous, but Final Fantasy XV -The Dawn of the Future- elaborates further in the realm's workings. According to the novel, when the corrupted Ardyn touched the Crystal 2,000 years ago, his soul became trapped in the "beyond", giving Ardyn an immortal existence akin to Bahamut's. Since Bahamut exists both in the physical realm and in the beyond, he can't be killed or infected with the Starscourge. Thus, Bahamut needs a True King to descend into the realm to destroy the corrupted part of Ardyn.
The True King's way to enter the beyond is to sit on the throne of Lucis under the Crystal and use the Ring of the Lucii to summon the kings of yore to impale him with the royal arms. To fulfill the prophecy, Noctis kills Ardyn as a mortal, and then joins the past kings in the beyond where they find Ardyn in a void. Noctis and the Lucii destroy the Starscourge within him for good before disintegrating into glimmering light particles. Lunafreya Nox Fleuret's spirit also appears in this realm to reveal Ardyn's true form before Noctis commands the Lucii to attack. When Lunafreya died in Final Fantasy XV: Episode Ignis, her body vanished into thin air with her brother lamenting that she will not rest until the prophecy has been fulfilled.
In Final Fantasy XV -The Dawn of the Future-, Bahamut revives Lunafreya with a new mission: fell Ardyn and gather his darkness to empower his Teraflare that will obliterate life on Eos. Noctis, Ardyn, the Lucii and the other Astrals cooperate to fell Bahamut in both the mortal plane and the beyond to destroy him for good. While Noctis and the other Astrals fight him on the mortal realm, Ardyn uses the Ring of the Lucii to enter the beyond with the Lucii to kill Bahamut's spiritual self. As Bahamut dies, the Crystal shatters after absorbing all remains of the Scourge from Eos, cleansing the land.