The Inciting Challenge

Here's a bit of unused content for another game.

Is it a blessing? Is it a curse? A gift from the gods? A mutation? A natural part of the world that anyone can invoke if they know something's True Name?

I don't know. It's your problem now.

The Inciting Challenge works as follows:

For all targets: You invoke The Inciting Challenge by invoking a character's True Name and issuing a challenge to fight. That character must make a save vs. spells (or Will, or Wisdom, or the local equivalent) at -10. If they fail, they are immediately overcome with rage and wish to kill The Challenger. They can still act intelligently, but automatically fail any actions that do not involve attacking or preparing to attack The Challenger and they will not willingly leave sight of The Challenger. Conversely, those affected by The Inciting Challenge gain a +5 to attacks and saves when fighting The Challenger. Once invoked, The Inciting Challenge is permanent unless undone by magic that removes curses, restoration or its equivalent, or any of the wish variants. Not even death revokes The Inciting Challenge; unless the corpse is sanctified or obliterated (and sometimes not even then), the decedent will often return as undead to attempt to kill The Challenger. If The Inciting Challenge is revoked, the target is aware it was magically manipulated. (The Inciting Challenge is otherwise not obvious, although an active detect magic spell can sense enchantment magic when The Inciting Challenge is invoked.)
     If The Challenger leaves the target's presence, the target will attempt to hunt down the target and confront them to the best of their ability. They do not have a supernatural ability to find The Challenger, but will usually devote all available resources to doing so. Additionally, if the target tells anyone about The Challenger, that person must also save vs. spells at -10 or else be affected by The Inciting Challenge against The Challenger.
     If The Challenger is slain, the target’s bloodlust ends, but it will return if The Challenger returns to life.

For non-player character targets: The target’s NPC attitude turns Hostile while under the influence of The Inciting Challenge, and cannot be modified by The Challenger or their companions under any means.

For player character targets: Player characters get a little more agency than NPCs, and so do not need to make a save vs. spells to avoid The Inciting Challenge. Instead, PCs get A Choice.
1) Ignore The Inciting Challenge.
2) Accept The Inciting Challenge. If the character accepts, they get the same +5 bonus on attacks and saves against The Challenger. They can take other actions without penalty. If they kill The Challenger, they gain a level immediately, gaining enough experience to be 1 XP away from the next level above that. (So a Level 6 character who kills The Challenger becomes Level 7 and is 1 XP away from Level 8.) This level gain is not subject to the one-level-per-adventure restriction of most old school games, so the character very well may gain a second level at the end of the session. If The Challenger gets away, the target automatically spends half the treasure they earn each session trying to find The Challenger until they find and slay The Challenger. If The Challenger plays in a FLAILSNAILS game, the GM should always grant a target of The Inciting Challenge a spot in the game if possible, even if session membership is usually random.

On True Names: In a pre-industrial society, True Names usually comprise a character's first and last name (or first name and title), although some characters may keep their actual names secret, instead going by an alias. (Although characters need to watch that, as an alias might become a character's True Name if it is more widely known!) Creatures such as gods, spirits, demons, and ancient dragons often exist in multiple planes simultaneously, and so have appropriately-complex True Names that are often obscure.
     The GM is the ultimate arbiter of what constitutes a creature’s True Name.