Orthogonal Attackers: Name-Thieves, Memory Tyrants, and Living Echoes.

Three Orthogonal Attackers

    What follows are three monsters that attack things other than your players hit points, in the vein of many classic dungeon encounters like the rust eater, level draining undead, and the aurumvorax. It is sufficient to propose that there are things the world can threaten other than a persons life or autonomy, but let's digest that concept a little more. Not all of these proposals will be well suited to all games for reasons of both design and personal taste.

    Name-Thieves

Living embodiments of unreality that hunger to be something, torn away from the Abyss or the Black Ocean or the plane of Shadow or the restless civilian dead, these tangible but indistinct figures crave meaning. They crave purpose. The crave a name. One might encounter them in any dark, sad or lonely place. Sometimes (on a high reaction roll) they may be reasoned with, and offered a new name, or the name of a favored weapon. But often (middling to low rolls) their craving for meaning makes them lash out and reach for the names of others. They often speak, although sometimes in strange old tongues and dialects.

It is a sad fact of their existence that no name lasts long in their possession, only 1d12 weeks, at which point they are merely a thief again - defined only by the absence of what they crave. 

A Typical Name Thief

AC: As Chain (but see defenses), HD: 3** (11 hp), Attacks: 1 x name-seize (no damage but see special), THAC0: 16 [+3], Movement: 120' (40'), Saving Throws: D10 W11 P12 B13 S14, Morale: 6, Alignment: Chaotic, XP: 100, Number Appearing: 1d3, Treasure Type: None

Defenses: Only objects which have been named can touch a name-thief. This must be something more specific than a categorical or taxonomic description. Any weapon with a name or the unarmed attack of a person with a name will surpass these seeming insubstantiality. Spells are always named. 
Special: When a name-seize attack hits, the target must make a saving through versus Death. If they fail they must erase their name wherever it is written and may never respond to that name again in the future. As a result of this sudden shift, any prior relationships are destabilized, requiring new reaction rolls on future encounters. 

 Memory Tyrants

   

   One might posit by their name that Memory Tyrants are relatives of beholders, but it is difficult to say for certain. The mind slides of these dark drifting spheres like oil on water. The come to feast, to rule over the domain of memory. Perhaps their gaze (if they have one) snips the neurons hippocampus or perhaps some n-dimensional tendril drifts from their great bulk to graze the dreaming soul of those they feast upon. Regardless, to encounter one is many explorer's worst nightmare, leading to a death amongst the cavernous tomblike realms of the mythic underworld.

 Typical Memory Tyrant

AC: As Plate, HD: 6* (28 hp), Attacks: 1d4 x ?? (1d6 + flense memory), THAC0: 13 [+6], Movement: 150' (50') flying, Saving Throws: D10 W11 P12 B13 S14, Morale: 11, Alignment: Chaotic, XP: 650, Number Appearing: 1, Treasure Type: None

Flense Memory: The touch of the Memory Tyrant peels away memories. They particularly seem to savor the taste of spatial memories, taking those first and foremost. Every time they hit with their impossible to remember strike, the Referee should take up the map of the group and remove a portion commiserate with the damage done. It is ideal to do this with  scissors, but the method can be adapted to the Referee's taste.
Hunt Through Memory: Anytime anyone mentions the Memory Tyrant, even after it is dead, it may reach out and caress them. They must save versus Spell, or take a point of damage and have a single room removed from their map. 

 Living Echoes

    Living echoes often congregate around sirens or other particularly clamorous occupants of dungeons. They relish in novel sounds, although their taste may vary. One may love best the sound of new weapons clashing in battle, another new songs. Intangible and fluttering they collect the sounds into themselves and repeat them endlessly in an ever growing cacophony. They offer little direct threat to adventurers or other dungeon denizens and indeed, if tamed or coerced into behavior they could be entertaining or useful. 

    But for most adventurers they are intangible, annoying, and attractive to enemies. The presence of a living echo following a party doubles the chances of any random encounter occurring. Fortunately for most adventurers, these irritants are easily distracted, and will depart after 1d8 turns of silence from the party or 6d8 hours if no effort is made to remain quiet. 

 Typical Living Echo

AC: As Unarmored (but see intangible), HD: 1/2** (2 hp), Attacks: 1 x buffet (1d2-1), THAC0: 19 [+0], Movement: 240' (80') flying, Saving Throws: D14 W15 P16 B17 S18, Morale: 5, Alignment: Neutral, XP: 7, Number Appearing: 2d10, Treasure Type: None

Intangible: The living echo, being made of sound, is intangible. It may be harmed with magic weapons, which disrupt the thaumatic fields binding it. Or so is the theory. They must also save versus death when exposed to a silence spell. 
Clamour: A living echo causes random encounter rolls to occur on double the chance. Additionally, in a group they may increase their volume such that all present are unable to communicate with words and must save versus spell or lose the ability to listen at doors for 1d4 days.