Niobe, Regina di Tebe, Royal Opera

What theatrical capital there is in the miserable little tale of Queen Niobe is not immediately obvious. The story crops up in The Iliad four pages from the end. Achilles summons up the sorrowful queen, while consoling the grieving Priam, as if to say, "If you thought your life was bad, you've got nothing on poor Niobe."

Niobe, Queen of Thebes, bears witness to the bloody murder of all 14 of her children, the consequence of a moment of overreaching pride. She is then turned to stone, and forced to weep for evermore. It is a tragedy of unremitting horror. In the Metamorphoses - the source for Steffani - Ovid is at his most Tarantino-esque in his recounting of the bloody fall of an entire family.

Steffani's 1688 operatic treatment fills out the back story with those other famous Thebans, Creonte (Iestyn Davies) and Tiresia (Bruna Taddia), reduces Niobe's total offspring to four, and washes out the blood. A fresh lowly foil to the central royal characters - in the form of the two honest lovers, Manto (Amanda Forsythe) and Tiberino (Lathar Odinius) - are introduced to offer some light. All of which fleshing out allows space for Niobe's character to develop and descend and for the drama to puff out its chest with pride.

To compete with the dramatic ebb and flow, a sort of magic emanates from the pit, as Thomas Hengelbrock - who was responsible for rediscovering the work and producing it for the 2008 Scwetzingen Festival - and his own Baroque orchestra, Balthasar Neumann Ensemble (who, technically, put most of our British Baroque orchestras to shame) pull more and more new sounds out of their orchestral hat. There's the hellish wheeze of a raspy medieval regal (a sort of advanced squeezebox), the doleful shy breeze of a recorder, the strum of a guitar, the rattle of a tambourine, trumpets, viola da gamba, timps; all phrased to sweet perfection.

It is a magic that is more than matched by Steffani's magpie style. His arias, mostly da capo, flirt with French dances, march with Purcellian ground basses, stun themselves into ravishing slow rounds and canons, allowing the strings to lap against their sleepy heads, like Handel would do 20 years later with more than a nod to Steffani. Arias duet with recitative; recitative court ariosi; tempi morph mid-aria and, periodically, there is the most heavenly slippage - chromatic contrary motion - that had me reeling. It adds up to a compositional style that has more freshness, economy and clarity than any of Steffani's contemporaries.

Director Lukas Hemleb's focus, too, was on economy and clarity. Most of the time a simple, nicely stagy Victorian vision of Greek mythology, one full of familiar archetypes - black-winged emissaries from Hell, gold-breast-plated gods, crowned Kings and Queen - prevailed. But every now and again a fantasy took over. A glitter ball offered us a magical starry canopy all around; a Boschian, black Lycra sack of bodies spewed forth the dark arts of Polifermo and his charge Creonte; a huge, weightless room of balloons, bouncing languidly off the walls like a Warhol installation, carried us into the heavens. There wasn't too much flapping. The story was told, the touches of surreal genius were delivered. One couldn't ask for more.

And there was one more revelation to be had: the male soprano voice of Jacek Laszczkowski. A truly astonishing range, facility and power in the very highest registers was to be found in his efforts. Not all of it was as elegant as countertenor Iestyn Davies's controlled, carefully phrased and ardent contributions - and certainly Davies won the clean coloratura competition - but Laszczkowski still stole the show with his soulful and soft thoughts of heaven at the end of Act One, a ravishing aria, delivered in the most exquisite sotto voce, and his barnstorming ascension into heaven in Act Two.

Véronique Gens's Niobe is given a less interesting vocal role in many respects, though psychologically there is plenty for her to do. Gens cut an equivocal baddie. She hit the heights in her final offerings, where Niobe has to endure divine wrath. The two other females added much to the mix: Delphine Galou's Nerea, ripe and lively in voice, the harbinger of the end-of-act saltarello, was a handy jester, while Amanda Forsythe's dewy, bright Manto (one to watch) brought a frisky spring to the Theban step.

But the honours must go to Hengelbrock, not just for rediscovering this work and somehow convincing Covent Garden to take it on but also for making what might have seemed at the start to be a three-and-a-half hour slog a perfect joy. I've never heard a Baroque orchestra roared on so loudly by a Covent Garden audience. They thoroughly deserved it. And they thoroughly deserve your custom. For this is without doubt the operatic revelation of the year.