I couldn't miss the chance to practice my drawing!
Nil is a power of nothingness in Final Fantasy Dimensions and a major element serving a major plot component, but is not seen or known of until late in the game.
Contrary to other incarnations of the Void in the series, where the Void is not malevolent in nature, the power of Nil is both sentient and malignant. It can create dangerous monsters of strength far beyond the regular monsters, known as the Armies of Nil. The strongest of them are known as the Blades of Nil, and are composed of the monsters Suffering, Despair, Anguish and Misery, whose sole duty is to bring evil to any human settlement absorbed within the Void, and can nullify or prevent positive human emotions, such as Trust and Love among others. These monsters are known to be sentient and perform their duties following the Nil's will.
Nil can grant its user massive power, as Elgo acquired most of its powers from the Nil and was unharmed by the Nil for an extensive amount of time despite is malignant nature. As Elgo extracts more power from Nil, it gives him access to several powerful spells such as Pandora's Box.
It is hinted but not outright stated that the Divine Generals of the Avalonian Empire are holders of the power of Nil, although to an unknown extent, as Shango after his defeat confesses that he used to be a human, much to the shock of the warriors. After her defeat, Cocytus expresses surprise that "humans" could defeat her, hinting that despite her more human appearance, she is a monster. The best example lies within Imperio who possesses Holy powers and looks human, but becomes a shadowy monstrous figure when he transforms into Gehenna with his powers changing to Dark powers. During this time Gehenna's shape resembles the extensions of Nil that invade the world.
It can be concluded the power of Nil can vastly increase the power of a person but at the cost of turning them into monsters. The generals Asmodai, Baugauven and Styx were all resurrected and turned into monsters with their strength far surpassing their human selves. Although it is never directly stated they were resurrected by the power of Nil, this is likely, as Elgo had gained control of Nil at the time and the generals were fought in sections of the Void where Nil resides. The Divine Generals and Elgo are fully aware of their actions while holding the power of Nil, but the lower ranked Generals lose all humanity.
Despite being sentient, Nil itself has no shape or form because it is a force of nothingness, and as such it cannot be harmed or destroyed, unless it takes a proper shape such as taking a host.
The power of Nil is seen after a Dimensional Shift occurs and re-merges the Worlds of Light and Darkness into the World of Dusk. The fusion is not perfect, causing several pieces of land to be missing and creating several portals known as Dark Flows leading to segments of the Void. The Warriors of Light and Darkness travel within these areas to acquire Dark Matter, a source of power as strong as the crystals, before the Avalonian Empire and the resurrected, now demonic Generals can.
After acquiring the pieces of Dark Matter, the barrier surrounding Castle Avalon is lifted and the warriors assault the stronghold, battling and defeating the Divine Generals, Cocytus, Shango, Imperio and Gehenna. After defeating Gehenna the warriors confront the Emperor, only to discover an empty suit of armor, and a portal to the Void opens. The warriors venture into the Void, finding within it towns it has absorbed, their inhabitants devoid of positive emotions due to the effects of the Blades of Nil, powerful monsters born from Nil itself. Defeating the Blades - Despair, Anguish, Suffering, and Misery- they return to the people their stolen emotions, also causing a major concentration of nothingness within the Void.
Though the warriors attempt to make their way forward, pitting themselves against Dark Flow, they cannot make any headway until Vata appears, revealing that as Nil is nothingness, it cannot be harmed. Fusing with Dark Flow to give it a body that can be defeated, Nil becomes Nil Vata, whom the warriors defeat, Vata passing on to them his wish to defeat the Empire for destroying his town.
The warriors venture into the World of Nil, the deepest part of the Void, full of powerful monsters, until they reach a castle-like area and find a shadowy figure seated on a throne which reveals himself to be Elgo. Diana and the Warriors of Darkness, who traveled with The Mask, express their doubts as to Elgo's true identity, but he reveals the Mask was indeed his "other half," the personification of his doubts and what good remained, which he considers flaws.
Elgo, revealing himself as the Avalonian Emperor, states he desired the power of the Crystals to cheat death and rule for all eternity. Revealing he had prophesied that the Crystal would break, he was fully aware the world would split and re-merge, but all was within his plans, as the power of Nil allowed him to travel between the Worlds of Light and Dark, manipulating the Warriors of Light into collecting the Crystals for him. Stating he will honor the Warriors as "heroes of the new era", he demands the Crystals, but the Warriors refuse, and Elgo battles them using the power of Nil, claiming it to be absolute. As the battle continues, Elgo extracts more and more power from Nil, gaining greater strength and new spells, but the warriors hold their ground. Eventually losing control, Elgo is engulfed by Nil, consuming him. Nil manifests in the World of Dusk as Dark Flows and monsters appear, but the many companions the warriors met on their journey encourage them not to give up.
The Warriors venture into the core of Nil where they find Elgo transformed into Chaos Elgo, an incredibly powerful monster with power over Light and Darkness. At first the warriors attempt to fight him, but as Chaos Elgo unleashes a Dark Flow on the warriors which almost defeats them, Barbara, Gramps, Eduardo, Sigurd, Argy, Matoya, Jinnai, Gawain, The Mask and Sophia appear in spirit to help the warriors hold Chaos Elgo back while restoring the warriors to full strength. The warriors battle and defeat the monstrosity once and for all.
Nil is still unstable and the abundance of light absorbed is about to explode. The twins Alba and Dusk stay and hold back the explosion for the others to escape. Glaive also decides to stay, trying to do something for his friends while accepting Diana will not return his love. The spirits of their deceased allies teleport them away and the world is returned to normal with Nil disappearing into the Void once more.
Nil is a word commonly used to mean nothing or zero; it is one of several names for the number 0.