Planar Bestiary
Law of Self-Cancellation: If a creature meets a parallel version of themself from the Planes of Mirror and Shadow, one or both could become obsessed with killing the other. In some cases, that psychosis spreads to other, unrelated creatures, until finally whole dimensions could be extinguished if the spread isn't stopped.
A parallel version of a character, bedecked in all manner of strange items from hundreds of different dimensions, tracks them down across the multiverse to kill them.
A convergent killer may have decided to track down all other versions of themself because of overweening narcissism. They may do it to subsume the power of all their alternates. Or perhaps an alternate version of the character—or the character themself—mucked with magic that briefly produced a parallel version of themself, thus invoking the little-understood and rarely encountered Law of Self-Cancellation. In any case, the convergent killer is the result.
Motive: Assassinate alternate versions of themself
Environment: Anywhere in the multiverse, usually alone, or as a group targeting a parallel group
Health: 25
Damage Inflicted: Moderate wound (5 points) (7 points when targeting their counterpart)
Armor: 2
Movement: Short
Modifications: Disguise (as their dimensional counterpart) as level 7
Combat: The killer attacks twice each round, once with a weapon taken from another plane of existence (such as a pistol or pet monster), and once with a duplicate signature attack, ability, or artifact of the character they have targeted. Attacks, defenses, and other tasks opposing their dimensional counterpart are eased by two steps.
The killer regains 5 points of health each round while their target is within short range. If the killer eliminates their target, the killer is completely healed and rejuvenated. The eliminated dimensional counterpart's body turns to dust, and as their action the killer can use the liberated energy to move to another world of the Material Plane, where another parallel version of them exists.
Interaction: A convergent killer often attempts to pass themself off as their dimensional counterpart to learn useful information, and later, sow confusion amongst their target's allies.
Use: A character activates a forbidden artifact, creating a doorway through the multiverse that allows a convergent killer through.
Loot: The convergent killer often possesses a mix of two or three cyphers that the target character has previously owned, and possibly even an artifact the target previously (or currently) owned.
GM Intrusion (Group): The convergent killer teleports up to a long distance, appearing next to their target. All other creatures within short range that fail a Speed defense task are roiled with dimensional instability, teleported in random directions a long distance away.