A Gentle Ending, A System-Agnostic Spell

 A Gentle Ending

Description:  A complex magical formula which requires great skill to master and impress in memory, but easily cast with just a subtle gesture after doing so.  The magic is quite specialized and not widely known.  The few groups that do employ it are often quite proprietary about ownership, in part because they know they're likely to be the first to be suspected when the spell is used.

Effects:  This spell targets a single living creature within arm's reach (5 feet if using a grid map).  Casting is nearly instantaneous, silent, and may easily be overlooked by observers, and does not make the caster vulnerable to melee attacks as many other spells do.  If the target fails to resist the spell at a -25% penalty, a complex magical resonance begins to build within their body which will strike them dead in 1d6 x 1d6 minutes.  

There are no obvious signs of the target's impending doom before then, and they are unlikely to realize the danger without magical assistance.  When it comes death is sudden, painless and the cause cannot be determined by mundane means.  Even magical methods require a contested check to determine that there was magic involved, and even then the exact spell involved is not apparent unless one is familiar with it beforehand.  

The resonance can be detected by appropriate magic or mystical senses, and may be dispelled with suitable counter-magic at any point before death occurs.  

Possible Origins:  A Gentle Ending is widely held to be the invention of some order of assassin-mages, but exactly which of the many shadowy organizations of arcane death-dealers that plague the world is the subject of much debate.  Suspects include the Cult of Violet Shadows, the Unspeaking Servants of the Qing of Quun, an nameless sect of Farthest Kuth that worships an antediluvian serpent-god, the Left Hand of the Prince of Shadows, the Right Hand of the Prince of Shadows, the Hidden Hand of the Prince of Shadows, Wononivan's Spellslayers, and at least six members of the Seventh Circle of the Guild of Quiet Dying.  And those are only the ones known to have used the spell, which by its very nature is easily mistaken for natural deaths, acts of the gods, or sudden psychic assault.

Complications:  Using this spell openly is a good way to earn a reputation as a sinister magical assassin, which may result in numerous unexplained deaths being attributed to you for better or worse.  Perhaps more dangerously, the types of people who regularly use A Gentle Ending are, in fact, sinister magical assassins, many of whom regard the spell as their private property and don't appreciate amateur outsiders using it.

There are also a surprising number of people who object to the use of magic specifically designed to secretly murder people.  Many of them own torches and pitchforks and regard mob justice as a reasonable response to inadequacies in the local legal system.

Design Commentary:  If you're playing a game that uses D&D-style magic, A Gentle Ending should be about 6th level, same as the pragmatically-named Death Spell.  A regular Death Spell is longer-ranged, enormously better at killing multiple targets, works instantly, and generally much better in a fight.  This spell is a specialized assassin's tool.  Subtle, effective even against very powerful single targets, and with some time to make a quiet exit and establish an alibi baked into it.  It's not much help when dealing with enemies trying to kill you right this very moment, though.