Cadaver collectors are like monstrous Roombas that scour the endless battlefields of Acheron, scooping up corpses and recycling the souls that formerly inhabited them into specters, which are then bound to fight for the cadaver collectors’ masters. Although native to that outer plane, they can be summoned to other planes as well, including the prime material—and if the summoner dies or loses control of them, they just keep on Roomba-ing around, turning living beings into cadavers if there aren’t already cadavers handy.
With Intelligence 5, they’re mechanistic juggernauts that never vary their method. What is their method? Well, they’re brutes, with extraordinary Strength and Constitution, so whoever, whatever and wherever their targets may be, they march straight at them. They have no independent judgment and lack the Wisdom to discriminate among targets, but they have a function, and a good machine completes its function with maximum efficiency, so they tend to head toward concentrations of bodies, whether those bodies be alive or dead. When they’re close enough to three targets to engulf them all in a 30-foot cone of Paralyzing Breath, that’s what they do (as long as this recharge ability isn’t on cooldown). And when they come within melee reach of a paralyzed target—or when cheeky opponents with magic or adamantine weapons run up and impertinently attack them—they employ their dual Slam Multiattack (rolling with advantage if the target is paralyzed, with every hit a crit).
What if they’re attacked by foes with nonmagical, non-adamantine weapons? They ignore it and keep juggernauting toward the nearest knot of humanoid organic mass. These attacks can’t hurt them and have no relevance to their mission. Woe betide the third opponent to get in on the action, though: at that point, the cadaver collector’s density-detecting algorithm kicks in, and all it has to do is take a couple of steps back—heedless of opportunity attacks—to nail all three with Paralyzing Breath.
Once per combat, a cadaver collector has access to the Summon Specters bonus action. The question is when it uses this feature, which has no particular connection to how many opponents it’s fighting or has already defeated in a given combat encounter, and I’m not sure that any obvious answer to this question presents itself. One could reasonably argue that, for maximum advantage, it always uses it in the very first round of combat, to maximize the difficulty of the encounter from the get-go. But am I alone in thinking that’s deeply unsatisfying from a narrative point of view? It seems like Summon Specters should happen at a more dramatic moment, doesn’t it?
So what could that moment be? Should it be tied to how much damage the cadaver collector has taken? (Having taken moderate damage—being reduced to 132 hp or fewer—would be a reasonable bar to set for this criterion.) But the cadaver collector is a servant—nay, an implement. As a construct, it has no self-preservation instinct and no purpose other than to follow its instructions. It seems more like the trigger for Summon Specters should be connected to what’s happening with the cadaver collector’s allies. But then again, there’s also the possibility that the cadaver collector has no allies, because it’s gone errant.
This is going to be a real reach—I admit it forthrightly—but what if the cadaver collector has a sort of uncanny intuition for when the tide of battle needs a nudge, either when the introduction of some fresh forces could turn a losing fight into a winning one, or when a battle the cadaver collector’s side is winning with great effort could be turned into one it can win handily? If you need to make this sense weirder and more mechanistic, what if the cadaver collector always bases its judgment on however many allies it had, and whatever condition they were in, when it was last under its summoner’s control? In other words, maybe it was summoned as an auxiliary to the Khuren Kharuul, an elite guard unit 50 hobgoblins strong, and it activates Summon Specters once combat has gone on long enough that it’s strange that its opponents would still be holding out against 50 hobgoblins.
None of these rules really lends itself to consistent application. I think the best rule is probably to use Summon Specters when it feels right to you as the DM, and not to try to justify it.
It should go without saying, but just in case it doesn’t: A cadaver collector doesn’t retreat, no matter how much damage it’s taken. It does its thing until it’s busted.
Next: steel predators.