The Last Word

The Last Word is a spell known to the most wicked arcanists and the most evil gods. It is a fundamental truth of reality, but one hidden in darkness and locked behind wards, barriers, ancient laws, and entire orders of devoted defenders. It is the darkest spell that exists, meant for only one purpose: the extinction of all life. It is wildly difficult to cast, requiring years of research and planning, fabulous sums of gold, and the sacrifice of many lives. It must be gathered together, each piece of the ritual discovered and catalogued, each syllable of the incantation carefully notated. The caster must be warded against being killed by the spell in the casting, punished by the gods for the deep evil it unleashes. The magic the caster spends to wreak this havoc is the least part, a mere binding seal in the form of a single word.

The rites to cast the Last Word are long and complex. If they are interrupted by a period of strenuous activity — at least 1 hour of walking, fighting, casting spells, or similar adventuring activity — the material components and invested spell slot are lost, and the spell fails. When all of the rites have been successfully completed, the caster who focuses the magic gains the Last Word, which they can speak as a reaction at any time in the following year and a day. If the word is not spoken within that time, the casting is lost. To regain the Word, the casting must be repeated.

The spell's effects are as follows:

• The Limits of Evil. This spell can only be triggered on the plane where the rituals that empower it were performed. The rituals automatically fail if performed on any of the Outer Planes, as the strong divine or infernal magic of those places denies the power of the Word. 
• Depravity's Consequences. The rites to create the sealing word are grueling. At the end, the caster has five levels of exhaustion and must make a DC 17 Constitution saving throw or fall unconscious, though they immediately stabilize if they collapse.
• Eternal Silence. The Last Word is aptly named. If it is spoken, the caster loses the power of speech and is stricken mute forever. Not even a Wish can return the ability to speak.
• Alone Forever. The caster is otherwise immune to the effects of this spell. They remain untouched.
• Death To All. The magic of the Last Word unravels all life and undeath on the plane of reality where it is spoken. Every creature of every type falls dead at the same moment, across the entire plane. Only the caster remains, and anyone protected by the Feign Death or Sequester spells. A creature may also protect themselves from the Last Word by means of the Wish spell, with the proper wording.
• Quiet. After the Word takes effect, the world is sterile, but not dangerous. There are no lingering effects beyond the simultaneous extinction of all life. Any travelers to the dead plane may travel there with no consequences, as the Last Word's magic has been entirely expended with the pronouncement of its seal.