Eidolons are intriguing creatures, because despite being undead, they’re not necessarily evil—they may even be good. Spirits honored by the gods for their zealous devotion, eidolons spend their afterlives guarding those gods’ sacred places and protecting them from defilers. Their compulsion—which every undead creature must have—is to protect. Not necessarily a bad thing!
Even more intriguing is that eidolons can hop into inanimate objects and animate them for the purpose of carrying out their eternal mission. Mordenkainen’s Tome of Foes offers, as an example, a stat block for an animated statue.
But first, let’s look at what an eidolon can do on its own. The flavor text says, “An eidolon has few methods for protecting itself beyond its ability to awaken its sacred vessels.” How true is this?
It’s true that a “loose” eidolon’s physical ability scores are all poor. Its mental abilities, however, are robust, with an extraordinary Wisdom score leading the pack. But by reverse-engineering the eidolon, we can determine that it has a +4 proficiency modifier, which in turn tells is that its Divine Dread ability is driven by its Charisma, not its Wisdom; otherwise, the save DC would be 16 rather than 15. So there’s a little dissonance there.
Mainly, the eidolon’s Wisdom serves as a defense against any attempt to manipulate it through magic. It has proficiency in Wisdom saving throws and can’t be charmed, exhausted, frightened, grappled, paralyzed, petrified, poisoned, knocked prone or restrained. (That leaves blinded, incapacitated and stunned as three conditions it is vulnerable to. Also deafened, but that’s not nearly as much of an inconvenience as being blinded.) It also has advantage on saving throws against being turned.
As for physical damage, it’s resistant to acid, fire, lightning and thunder, along with physical damage from nonmagical attacks, and it’s entirely immune to cold, necrotic and poison damage. That leaves force and radiant. Yay, sacred flame and magic missile! You are valuable!
So the eidolon is pretty good at not getting hurt; what it’s not good at is hurting anything back. Aside from animating a statue or other vessel, its only offensive ability is the Divine Dread action, which can impose the frightened condition and carries the common rider that frightened targets must hasten away. Not only is the save DC not all that difficult to hit—someone in the party is bound to make that save—the effect is temporary, lasting only one minute, and either success on the saving throw or expiration of the effect confers immunity to it. In the grand scheme of things, the only thing this feature does is buy the eidolon time to board its vessel.
While animating a sacred statue, the eidolon becomes a brute, with extraordinary Strength and Constitution (though its Dex is still pitiful). It retains all its damage resistances and immunities and most of its condition immunities, but it can be grappled, restrained or knocked prone in its statue form.
As a brute, the statue has a choice between a slam/slam Multiattack and throwing a rock (or any other handy, heavy object, I’d say) from a distance. The Multiattack is the clear favorite, doing an average of 86 points of damage (!!) on two hits vs. 37 points of damage from a thrown object, although the latter offers an excellent chance of breaking a caster’s concentration on a crucial spell. I don’t see the eidolon using its Rock attack for any other purpose, and an animated sacred statue has no other feature to make its tactics any more sophisticated than, “Rrrrahhhh, bash bash bash.”
Because of its compulsion, the eidolon won’t flee or retreat as long as whatever place or thing it’s protecting is threatened, no matter how much damage it or its physical vessel sustains. Note that the eidolon has one reservoir of hit points, and the sacred statue has a separate reservoir of hit points. When its sacred statue is destroyed, an eidolon no longer has a vessel to inhabit and is forced out, but it still has however many hit points it had before it animated the statue.
At that point, although there’s very little its opponents can do to it, there’s virtually nothing it can do to its opponents—especially if they’re no longer affected by its Divine Dread. All it can do is linger, impotently shouting at trespassers in whatever language(s) it knew in life, which has to be the saddest combat encounter dénouement of any officially published monster.