Some of my favorite books are the Thomas Covenant novels of Stephen R. Donaldson. For some reason, for a long time I thought the entities and forces in the "other world" of the story were meant as exemplars of Jungian archetypes, but I later found out the Jung connection is actually pretty weak and instead the author intended the other-world beings to be understood along the lines of Platonic Forms.
The author does a really good job of describing how an actual "moral sense" would work, which you would think is explained in the story as a side-effect of being in the presence of Forms, but that's not quite it, and I digress. More surprisingly vs. Plato himself is that the story revolves around a conflict with an entity that is more or less the Form of Evil.
So I did a short look-through of Google search results for "Plato Form of Evil" some time ago, and the few detailed answers I read were all like, "There's no Form of Evil because evil is a deficiency and the Forms are all perfect." And they seemed to think that this was Plato's sense of the "situation." I had no reason to disagree, because for all I knew, Plato never talks about a Form of Evil as a real factor in his theory.
Today, I searched for "Plato Form of Destruction" instead, though, and got a much different answer. This essay, near the end, talks about a Form of Death and either the same Form under another name, or another such Form, of Destruction.
Now, this can't be a Form of Evil on the grounds that death/destruction are exemplars of changing imperfection, with such change and imperfection being metaphysically evil. If every Form participates in itself perfectly, then the Form of Imperfection perfectly participates in itself if and only if it doesn't perfectly participate in itself (this is Russell's paradox before the fact, I suppose). So if the Form of Destruction is the Form of Evil, it isn't because of metaphysical imperfection.☠
Going back to the Covenant story, the author actually has four "divine" Forms in his system: Creation, Love, Indifference, and Despite (the "technical" term for evil in the story). It's a constant refrain in the tale that, "Love is not the opposite of Despite," or, "Law [meaning a blend of aretaic natural law and scientific laws of nature, and caught up in the power and will of Creation] is not the opposite of Despite." In fact, AFAIK, the author never says what the opposite of Despite is as such.
And the closest thing to a Form of Destruction in the story, as a Form of Death in particular (the Laws of Life and Death overall being the overarching natural laws of nature(!)), is portrayed as in itself morally neutral. Its awakening is a cataclysm, the final cataclysm of the narrative in fact, and instigated by Despite (through the manipulation of the protagonists), but Donaldson's Form of Destruction simply is not his Form of Evil.
I wonder, in the end, if Despite is actually its own opposite because Donaldson wants to say that evil is self-defeating, ultimately, but I'm not sure. The contingency of the protagonists' triumphs throughout the ten books is sharply emphasized. If evil has a built-in inevitable self-destruct sequence, such contingency would be less evident, I suppose.
Anyway, the Form of Creation might be opposed to the Form of Destruction, but the Form of Despite was actually entangled with Creation in the beginning, and posed as if it were Creation, seducing the Form of Love and betraying Her, leading Her to become Incarnate as Hatred. If that doesn't sound weird enough, the Form of Hatred is then redeemed by merging with transgender genius goblins in an underground city made of liquid artwork. On Her way out, She gut-punches the human Incarnation of Despite, laying him low so that the protagonists can then absorb him (not destroy him) and use his knowledge combined with their powers to rebuild the world after the Form of Destruction unmakes the Form of Time.
In Plato's world, every Form participates in the Form of the Good, not to be the particular Form that it is, but to be a Form at all. This is because auto to agathon is also the Form of Forms. So a Form of Evil participates, nonetheless, in the Form of the Good. Yet how then can it be evil? We seem pushed, again, to evil as ephemeral or even epiphenomenal (morally).
For all that, then, I think an unanalyzed Form of Evil is not given in Plato's system. But if there is a Form of Destruction...? I get the 'feeling' that such an entity would be 'midway' between Satan and Ahriman in terms of its power.☬ As a Form, under the Good, it must have the potential to be good. But suppose goodness requires free will. If the Form of the Just is the most committed to justice (besides the Form of the Good), then one imagines that this Form is what it is in part because it chose to be so, somehow. Likewise, the Form of Destruction would have to have used its free will for evil, or in an evil way, and this is how it is also the functional equivalent of the Form of Evil.✞
Does he or doesn't he (Plato) have a Form of Evil in his theory?
☠Likewise, there must not be a Form of Nonexistence, since if this Form participates in itself, it is an exemplar of nonexistence, i.e. it does not exist. Then the Form of Destruction, though it renders something unto nothing, is not itself the Form of Nothingness, after all.
☬Suppose that every Form is infinitely powerful. Then the Form of the Good is "greater in dignity and might" than all other Forms just in case there are at least relative and absolute infinity, wherefore auto to agathon must be absolutely infinitely powerful, the other Forms (waiving the complication of the Form of Power proper) having power equal to various aleph numbers. So one expects that the Form of Evil's true power is limited in some way (certainly it is not equal to the Form of the Good). Ideally it would be as limited as possible, i.e. perhaps to ℵ0. Now, if the seal on Evil's power were contingent, though...
✞Elsewhere, I've speculated that the Form of the Good is actually the Form of the City (or Republic) itself. Perhaps then the Form of Tyranny would be the Form of Evil. But if tyranny is spiritually destructive of a city, then...
My toy axiomatization of Plato's Forms had fundamental predicates for participation and for worlds. So there was a world of exemplars w and a world of Forms W. The 'upshot' was that w participated in W as a whole, but then W was itself a Form, and in fact had to be D = the Form of the Good, too, so that D = W. But Plato compares D to the sun rather than to what the sun illuminates (other than itself, so to speak). At any rate, departing from the solar analogy, if D is actually the Form of the City, then the other Forms are buildings that make up the City, and so D = W goes through after all. Moreover, it is no longer weird or unseemly for D to be first among equals in the sense of maximal participation in the other Forms, which maximum is otherwise granted only to each Form (so that the Form of Glory, for example, is the maximum of glory vs. all the other Forms except for D, since D inherits the Form of Glory's splendor inasmuch as that Form is a building in the City).
However, what becomes of the Form of Evil, here? Would it be another building in the City, like a weapons factory or execution chamber or some such thing? Or is it rather something that would destroy the City if it could, but which ends up capable of destroying only cities of exemplars in w?