R. M. KOSTER may have summarized the essential moral condition of Latin America since Christopher Columbus in a half-million-word metaphor that the future will call the Tinieblan trilogy. The condition is exploitation. Do it or be done to.

Outrageous, painful, struggling for coherance, such a summary logically would have come from a Latin-American conscience. But art acts on its own mysterious initiatives, and the Tinieblan trilogy -- The Prince (1972), The Dissertation (1975), and now Magdragon -- is the work of a gringo from Brooklyn who took to Central American in quest of his future. There R. M. Koster found myths, stories, anguish and comedy sufficient to create a world.

Koster calls this world the Republic of Tinieblas, and it's not a good place to raise kids or plan a future. In Spanish tinieblas means darkness, and the apocalyptic visitations Koster brings down on his imaginary banana republic are due to the Machiavellian power madness and sexual gluttony of his principal characters.

In The Prince Kiki Secundo, invalid son of a Tinieblan dictator, takes a Joycean inner voyage to explore the ruin of lives, resources and hopes in Tinnieblas. The Dissertation purports to be a scholarly biography, written by son Camilo, of another Tinieblan strong man, Leon Fuertes, who rose to power by way of gutter, baseball diamond, nightclub stage, and who excelled at soldiering and sex. Fantasy, dream, hallucination as well as steely reality stretched the boundaries of Koster's novels as he marveled at the waste of Tinieblas and wondered about martyrs and banality.

With Mangragon the work comes rightfully to conclusion with automatic weapons chattering, a massive earthquake tearing fissures in Ciudad Tinieblas, and a great human head spurting blood and smiling "a smile immensely tender, loving, gay" as it floats out to sea away from the exploding beaches. Koster's world ends with a bang and a smile.

Mandragon is the story of a hermaphroditic Tinieblan -- half-breed circus freak, cult leader, and supernaturally endowed liberator of the exploited -- and the final days of the Tinieblan republic. Like Tinieblas itself Mandragon betrays his wondrous gifts and destroys the country that has destroyed him. The source of these events is Mandragon's marvelous powers. They lie somewhere between those of God and Clark Kent. Mandragon reads minds and futures, flies through space, casts spells, collapses buildings and throws automobiles. Mandragon's arrest and decapitation, which frame the novel, result from his misuse of these gifts if order to murder Alejo Sancudo, yet another Tinieblan military dictator and possibly his own illegitimate father. Payoff for the murder is experiencing "for the first time . . . male gender, the essential happiness of love, which comes when one administers delight." Mandragon enjoys this pleasure through the services of Dona Angela Sancudo, the dictator's wife and vice president of Tinieblas who succeeds her dead husband, rakes in a fortune from a U.S. corporate crook, and then presides over Mandragon's execution.

Seldom did I believe that Mandragon could waft himself-herself to Manhattan to recruit flower children gone to seed, could command the skies to rain, or could switch sex from prison tart to palace stud. Nor did I believe that Mandragon's clown friend Rebozo had a trained Pomeranian named Bebe or that Dred Mandeville, Koster's version of Howard Hughes, cruised the deep for years in a pirated submarine -- the Scorpion, stolen from the U.S. Navy -- run by robots as he waited for extradition-proof asylum in Tinieblas.

But I believe the milieu: the sweating landscape, the human ignorance, the barbarous quest for power. Koster has caught the conquistador psychology that drove Pizarro and Cortez. He has imaginatively fashioned an alternative setting for the composite Trujillos, Juan and Eva Perons, Fulgencio Batistas, Fidel Castros and others who shout, plunder of fornicate their way through Mandragon . With a will to turn words into moods and conceits into possibilities, Koster also defines the circumstances of illusion. His downtrodden peasants and industrialized misfits believe in Mandragon's powers because Mandragon makes them believe in themselves. And we too fall for the illusions, if not for Koster's documentation of illusion.

While the most substantial moral claims I would defend of Koster's behalf -- in Mandragon , anyway -- are environmental or conditional, he himself has much to say about the morality of power: its accidental occurence, effect on possessors, causes of loss and sources of recovery. Some of this is serious: "Step forward into the mystery . . . calm but also vigilant, unwavering. Then it [power] may serve you even as you serve it." And much of it is tired. Like Samson, Mandragon "takes up service with the power of love" and it costs him his good magic.

R. M. Koster's fiction provokes extremes of praise or regret because his work itself moves to extremes. He bastardizes nouns into verbs like "fetusing," "shrapneling" and "phalanxed." He is addicted to lists, anecdotes and other narrative drugs. He toys with possiblities while his plot stands waiting.

But Koster is also born writer. He loves language and revels in stories. Every volume of the Tinieblan trilogy is an argument with fate. Koster feels human injustice and greed deeply. Unable to see how they can be changed, he ignites them with the fire of comedy.