I’m puzzled as to why certain creatures are included in the “Miscellaneous” Appendix A of the fifth-edition Monster Manual, which consists mostly—but not entirely—of regular animals, such as apes, bears, crocodiles and so forth. Why, alongside this menagerie of mundane beasts and their oversize cousins, do we also find awakened trees and shrubs, blink dogs, death dogs, wargs (excuse me, “worgs”) and phase spiders? Why didn’t these monsters (none of them is categorized as a “beast”) rate their own listings in the body of the book? So odd.
Phase spiders differ from giant spiders in a variety of minor respects and two significant ones. First, while they have the Web Walker feature, they don’t spin webs. This struck me as so peculiar that I checked the MM errata to confirm that it wasn’t a mistake. Second, they have the Ethereal Jaunt feature, which lets them phase back and forth between the material plane and the ethereal plane.
I’m going to take a quick look at its other traits and then come back to these, because I think the phase spider is in need of some flavor text that explains what it’s all about.
Phase spiders are brute fighters, with high Strength and Dexterity and above-average Constitution. With Intelligence of 6, they’re also cleverer than you’d expect a spider to be. Although they’re not clever enough to act beyond their instincts, those instincts can take them a long way: Intelligence 6 is equivalent to that of a chimp. It’s not unthinkable that a phase spider might even use tools. Finally, the combination of Stealth proficiency and darkvision makes phase spiders nighttime or underground ambush predators.
So we have an ambush predator that can navigate webs but doesn’t create them. Why the former ability and not the latter? And why the ability to phase back and forth between planes? Here’s my theory: Phase spiders didn’t evolve independently. Rather, they’re the result of a magical hiccup in the evolutionary process. Something created them out of regular giant spiders, and they continue to live side by side with their mundane sisters. In fact, that lovely cornflower blue illustration notwithstanding, I imagine that a phase spider is indistinguishable from other giant spiders until it begins blinking.
But wait a second. Spiders, by and large, are solitary creatures. In our world, there are tens of thousands of known spider species, yet fewer than two dozen of these are social. On the other hand, as a practical matter of gameplay, giant spiders cease to be boss monsters when our player characters hit level 2. We send groups of giant spiders at our players all the time. You’ve done it. I’ve done it. So I guess we have to say that in Dungeons and Dragons, giant spiders just happen to be one of those social species, or at least subsocial. And phase spiders, occasionally, live among them.
Fighting on their own, giant spiders will first seek to ensnare their prey in a web, if the prey isn’t caught already. Then, once the prey is restrained, they bite (with advantage) to paralyze. When the prey stops moving, they wrap up their treat to enjoy the leftovers later.
Phase spiders act in conjunction with this behavior, but since they don’t make webs of their own, they aid their sister spiders by taking out moving targets, gaining advantage as an unseen attacker by using Ethereal Jaunt to appear suddenly behind their prey. Phase spider poison is no stronger than regular giant spider poison (we can infer this from the DC of the saving throw, which is the same), but the phase spider delivers a lot more of it (we can infer this from the greater damage it does, 4d8 vs. 2d8). Thus, contrary to predators’ usual habit of primarily targeting the old, the young, the weak, the isolated and the oblivious, a phase spider may attack the larger of its opponents, knowing on some dim level that it has a better chance of taking them out than other giant spiders do. Plus, as a brute fighter, it’s more inclined toward aggressive melee engagement.
Phase spiders follow different action patterns based on whether they’re “blinking in” (ethereal to material) or “blinking out” (material to ethereal). When blinking in to ambush, a phase spider first uses its Ethereal Jaunt bonus action, then its Bite action. (To be fair to your PCs, the phase spider should make a Dexterity (Stealth) check, opposed by its target’s passive Perception, to determine whether it can appear behind the target without being noticed. If it fails, it loses its advantage on the attack.) When blinking out to escape a foe who’s hurting it, it delivers a Bite out of spite, then uses the Ethereal Jaunt bonus action to vanish.
What if a phase spider blinking in to attack successfully paralyzes its prey on its first strike? Then it uses its movement to bring itself closer—within melee reach, if possible—to another opponent who does fit the usual prey profile. Bigger opponents are for engaging only with stealth strikes.
Once it’s engaged with a target, a phase spider remains engaged with that target until the target is paralyzed or the spider takes moderate damage (10 points or more in a single round). If a phase spider takes that much damage in a round, it bites once more, then blinks out. When a phase spider is seriously injured (reduced to 12 hp or fewer), it blinks out and doesn’t return.