After a lacklustre trio of concerts traversing the seven symphonies, Sibelius' 150th anniversary celebrations at the Proms peaked with a gripping encounter with Kullervo, his early, category-defying epic based on the Kalevala. Neither symphony nor cantata, Kullervo is five craggy tone poems hewn from Finnish myth. Having a Finnish conductor as our native guide helped – Sakari Oramo, beaming like a benevolent schoolmaster and mouthing all the words – but the crowning glory of this performance was the singing of Finland’s Polytech Choir, joining the men of the BBC Symphony Chorus, who made thrilling, chilling choral contributions.

Kullervo only had a handful of performances during Sibelius’ lifetime, although the third movement “Kullervo and His Sister” was authorised for performance as a separate work. In 1957, after he had tinkered with that third movement’s orchestration, Sibelius decreed that the work could only be published posthumously. Kullervo had to wait until 1971 for its first recording (Paavo Berglund/Bournemouth SO) and 1979 for its first Proms performance (Gennady Rozhdestvensky/BBCSO).

Oramo was at his animated best on the podium, his gestures as extravagant as his waistcoat. He coaxed fine playing from the BBC Symphony Orchestra, although the string playing initially lacked the heroic sweep, the sense of being carved from granite. The introduction and “Kullervo’s Youth” (the second movement) establish our warrior’s troubled early days, largely spent in slavery to his wicked uncle. These entirely orchestral movements have their longeurs, which Oramo navigated with a practised hand. The strings found a glassy Nordic frost to their playing, while French horns had a well-rounded tone.

Woodwinds impressed with their runic-like interjections and in the squealing whines that punctuate the jolly march theme in “Kullervo Goes to War”. Sibelius also offers much triangle tintinnabulation to keep percussion busy.

It’s the two choral movements which really set Kullervo apart. Hearing the 130-strong choir, mostly from Helsinki’s male-voice Polytech Choir, launch into their sturdy narration of “Kullervo and His Sister” was unforgettable. The sheer volume of sound, the attack of a chorus singing in its native language, and the visceral impact of the singing ignited the whole performance. This movement relates the tragic tale of Kullervo who encounters a woman as he drives his sledge home having paid his taxes. Three times he tries to persuade her to join him, three times she refuses. Kullervo, refusing to take no for an answer, drags her aboard, rapes her and offers her countless treasures. During the commentary which follows, they discover that they are brother and sister, upon which the sister flings herself to her death in a whirlpool, while Kullervo laments his fate. Waltteri Torikka coped nobly with the roles high tessitura, especially given that his baritone is quite dark, while Johanna Rusanen-Karteno negotiated the Wagnerian range from gleaming soprano to smoky lower register fearlessly.

“The silence speaks,” Sibelius once remarked enigmatically. In the piercing brass calls at the end of the second movement, Oramo battled against the Albert Hall’s acoustics, the long delay meaning that the silence between them didn’t always register. However, his mastery of the silences in “Kullervo’s Death” was gripping. Further haunting male choral work graced this final movement, as our warrior returns to the scene of the crime to find the earth bare. There, he falls on his sword and dies.

How to partner such a unique work in a concert programme? Oramo chose to preface it with another early work, En Saga (literally “A Fairy Tale”) which set the tone for Kullervo which followed. After a measured start, and some uncertain woodwind intonation, the performance picked up, just as a storyteller grows more animated as the tale unfolds. A pulsating bass drum thundered out its motto (no role for the timpani in this symphonic poem), while husky flute and liquid clarinet solos painted the scene. At just under 20 minutes, it made for an economical, but effective opening half to this highly enjoyable Sibelius double bill.