This is the second production of a Sarah Kane play in London within just a few months and as we are freshly enthralled by her genius, the walkouts become almost like goals you want to cheer. Cleansed at the Almeida was a punishing, astonishing experience reminding (fleeing) audiences exactly why Kane has a fearsome reputation for extreme theatre (the list of trigger warnings in the foyer was Bayeux Tapestry sized). Now 4.48 Psychosis, her final play written shortly before her suicide in 1999 aged just 28, has opened in the Linbury Theatre at Royal Opera and takes a less violent but similarly traumatic path.

Given the experimental, elusive approach Kane took for 4.48 Psychosis – no defined characters or stage directions, long passages of self-coruscating poetry-prose, lists of medications, snatches of doctor-patient dialogue – it’s wide open for interpretation, and indeed feels almost uniquely apt for an opera. Composer Philip Venables and director Ted Huffman originally developed it for Royal Opera but it originally debuted at the Lyric Hammersmith in 2016 while the Linbury was closed for renovation. Ten years on it is at last showing in Covent Garden and is a devastating triumph. It touches on beauty and is always poetic, but it is terribly upsetting – the walkouts come as no surprise. But this is no mere shock parade, it puts a spotlight on terrible truths.

While six performers dressed alike sing in complex patterns of overlaying lines to represent a central character at war with herself, it all orbits around soprano Jacquelyn Stucker (she’s listed as Gwen in the programme and the other five have character names too, though they are not referred to as such in the text or in the performance). She is prostrate on the floor of the institution room set as the audience files in, and rises up to be put taken on a journey of turmoil and anguish, in one of the finest performances you’ll see on stage right now; utterly gripping to the last.

But the text is the star here, an internal voice locked into an inescapable, fractured doom loop. A voice which shifts between different forms in ways that are abrupt and extreme, as disruptive and disorientating as reality for someone falling apart. These shifts into new forms come fast and hard, and are often spectacularly achieved, as when the stage is lit up with numbers from a Serial 7 test (a cognitive test where a patient subtracts numbers from 100) accompanied by bursts of frenetic passages by chamber ensemble Chroma, who sit above the institution room. Jolting bursts of electronica interrupt calming swathes of clinic muzak, as conductor Naomi Woo rides a knife-edge of tension. The six voices break off into individual hells then rise together Ligeti-style in disturbing peaks of existential fear.

Most affectingly, the dialogue passages are ‘spelt’ out syllable-by-syllable by duelling percussionists, Catherine Ring as the patient, Delia Stevens as the Doctor. With so little characterisation on display, the pair beating out exchanges using hammers or bare hands or a saw, are as close as it comes to traditional scenes. It is a narrative of a patient at the end of their tether, disputing the assessments of the doctor, and the institution, and society that lies behind them. Dark humour emerges here. Illustrative arguments.

Patient: Do you despise all unhappy people or is it me specifically?

Doctor: I don’t despise you. It’s not your fault. You’re ill.

Patient. I don’t think so... I’m depressed. Depression is anger. It’s what you did, who was there and who you’re blaming.

Doctor: And who are you blaming?

Patient: Myself.

The internal voice that we follow is clearly Kane’s voice, given her own experiences with mental illness, and of course given what happened to her in the wake of its completion. While suicide hangs heavy in the air here, the play is not a suicide note. Instead it is a daring look into the abyss and a recording of what happens when it stares back into you. There’s a deep, horrifying loathing of bodily existence. Shame and guilt and hatred of the Self. A carefully rendered depiction of the warping of daily life that comes with deep turmoil. And an unflinching conviction that the act of living is painful. Kane dared to go down into the depths and report what she found there. Sadly, she did not make it back out herself.

This production is brilliantly accomplished but has its flaws. I am not convinced that the six person ‘split personality’ conceit doesn’t water down the trauma here, positing this suicide doomloop as a inner battle of warring selves, rather than the result of a rejection of the outside world and the brutality of a system that tries to ‘correct’ that. Unlike Cleansed, where pain wrought on the protagonists’ bodies is used to control their minds, here never really see the lead character victimised by the institution. Who would ever say: more horror on stage please! But a few punches may have been pulled here, in the context of her other work.

Nevertheless, in concentrating on the internal trauma this opera grips you hard and doesn’t let go. No ounce of sentimentality or self-pity are allowed into its suffocating space, with Venables resisting emotional release in favour of discordant avant-noise and expressiveness that is quickly snatched away. When his music does soften late on, it is only to underscore the irony of a fantasy sequence where the voice weakens and passively accepts life as presented in the outside world, telling it what it wants to hear. But this is never going to be her fate.

Bleak? Yes, it’s bleak. But Kane fans know there is hope here. There are suggestions that love could be an answer, because of its very absence. The same with trust, with compassion. If such things were to be given to someone else, they could be saved. Just not in this case.

This is an opera that will divide but which demands to be seen. This is no spoiler, but this reviewer was emotionally blindsided on the way out, after the performers had taken their bows, when the end section of Radiohead’s Exit Music (For a Film) played, with its closing refrain of ‘we hope, that you choke…’ The previous 90 minutes had wound something up inside and it suddenly snapped. And if you don’t want to be broken by art, what’s the point in experiencing it at all? To be soothed??

4.48 Psychosis is at Royal Opera until 25 October