Displacement Beast

Jumping on the elements blogging bandwagon, this was the first chemistry-dnd pun I came up with.

In displacement reactions, a more reactive element replaces a less reactive one within a compound (AB+C->A+CB). They're cool, exactly how I imagined science experiments as a kid. We had to learn the order of different metals' reactivities in school using a mnemonic about zebras, but dnd-type games are full of numbers that make it easy to work out the more reactive element in any situation. This monster cares about HD, but any number on a statblock could easily replace it.

Displacement Beasts look like a strangely behaving, more intelligent, something else. Until they open their mouths, revealing far too many teeth which hide a mass of thin floating strands grasping out where their tongues should be.

Stats as whatever creature they have most recently displaced, with the following changes:

• Intelligence noticeably higher than expected for the creature (e.g. wolf as smart as a monkey). If the form has language however, they are not very good at speaking (too many teeth, tendril tongue, always trying to hide their mouth).
• Disposition depends on whether they are encountered in the proximity of a higher HD creature. If yes, they are scheming to displace it. If not, roll 2d6

• Attacks against higher HD creatures can deal displacement damage instead of normal damage, tracked separately to normal damage. If displacement damage is greater than the target's current health (i.e. sum of normal and displacement damage has reduced the target to 0), it extends the mass of tendrils from its mouth to swap the spirits of the displacement beast and its target.
• Bite [HD/2]d4 damage, if the creature does not already have a bite attack.

Displacement Damage represents a targets soul and body disentangling as they begin to be replaced. Describe them gaining attributes of the displacement beast's current form and let this change the target's stats or behaviour during the course of the combat. Being reduced to zero knocks the target out for a few minutes, and they awaken in the body of the beast's current (now previous) form. A few minutes rest away from the displacement beast will remove this damage, but receiving a scar (or equivalent in your system) from dispacement damage results in gaining permanently an attribute of the current form.

Tactics The displacement beast always focuses the single highest HD target. As it is always fighting at a disadvantage, it relies on its intelligence and the element of surprise that its form grants it. It will use rudimentary traps, environmental features, and other creatures to try and reduce the target's health before attempting to displace it while it's weak. It is a delicate balance though, as the beast will need enough strength to get away somewhere safer before its target awakens in its previous body.

Encounter Design This could be an effective recurring encounter. The party notice some beast, say a wolf, trying to walk on two legs (it contains the displaced soul of an orc). Later on, they meet the orc tribe settled here and their companion disappears in the night, an orc approaching the party the next morning and claiming to be that person. The next time the party face the beast, it is uninterested in them- until eventually, they are higher HD than its current form. Now it searches them out, crude traps litter their established routes through the dungeon and when they encounter it they know exactly what they're getting into.

The displacement beast is very suited to a dungeon, where it acts as a kind of solo chaos faction, causing problems for everyone. The closed environment and clear areas mean it should be pretty obvious (to the referee and players once they work out what's going on) which creature will be the next target. It is also an easy monster to exploit. Lead a more powerful enemy into its domain and that enemy has a new problem. Trap some great beast in a humanoid body if you want to chat to it. Use illusion or conjuration to lure the displacement beast then capture it. Just remember it will always be looking to change form, and is very crafty. If the party become the target, make sure the displacement beast has done its best to stack the odds in its favour, any way that it can.

Treasure Capturing the beast is probably the best treasure, but if killed its tendrils can be harvested. They are surprisingly stretchy and when two creatures hold either end in their mouth, allow for perfect communication and heightened empathy between them (even if they have no other way to communicate). They could be used to craft psychic-themed items but are susceptible to decay. Wearing a necklace of displacement beast teeth protects you from possession and other types of soul swapping/stealing.