At the very end of Tristan und Isolde [Tristan and Isolde], Wagner introduces a new motive that has not been heard previously in that work and which is a key motive in Die Walküre [The Valkyries]. In all the commentaries, analyses and discussions I have consulted, none have pointed out this fact, let alone attempted to explain its presence.
I am wary of giving names to leitmotivs in Der Ring des Nibelung [The Ring of the Nibelung] and Tristan und Isolde. Leitmotivs are integrated into the texture of the orchestra and often derived one from another. Their meaning is not a simple linear relationship between one idea and one leitmotiv. Taking this into account, common names of leitmotivs (from other writers) are used below for ease of identification.
The leitmotiv as it occurs in Die Walküre is shown below:
In Die Walküre this leitmotiv appears throughout Scene 3 of Act 3. It is first heard at the start of Scene 3, played by the bass clarinet followed by the Cor Anglais [Dover score p. 598]. This first appearance is as the tail of the leitmotiv often named Wotan's Frustration. Brünnhilde's opening line in this scene then expands the leitmotiv into what has been labelled Brünnhilde's Reproach as Brünnhilde sings “War es so shmählich was ich verbrach” (Was it so shameful what I did wrong). These separate instances are one and the same music. It then appears in its definitive form and known as Brünnhilde's Compassionate Love when Brünnhilde sings “Der diese Liebe mir in's Herz gehhaucht” (This love in my heart) [Dover score p. 618]. It is heard again most emphatically after Wotan summons the flames to protect Brünnhilde from being claimed as bride by all but the greatest of heroes [Dover score pp. 663 - 667] as heard in the excerpt here:
These moments at the end of Die Walküre are poignant and heart wrenching. They work perfectly. None of that explains why or how the leitmotiv found its way into the end of Tristan und Isolde.
It is known that Wagner completed Das Rheingold [The Rhine Gold], Die Walküre and Acts 1 and 2 of Siegfried before setting aside Der Ring des Nibelung to compose Tristan und Isolde and Die Meistersinger [The Master Singers]. The leitmotiv of Brünnhilde's Passionate Love had already been composed.
Moments from the end of Tristan und Isolde, as Isolde sings “ertrinken, versinken” (to drown, to sink down), the leitmotiv of Brünnhilde's Passionate Love is played by flute, oboe and Cor Anglais [Dover score p. 652 - 653]. It is not always clearly audible above the E major chord in the other woodwind, brass, harp and tremolo strings. It is somewhat easier to hear in performances of the final “Liebestod” without voice, which is often done as an orchestral concert work, as heard in the excerpt here:
Die Walküre and Tristan und Isolde both feature strong female lead characters. They share the themes of sacrifice for love and redemption through love and sacrifice. Did Wagner incorporate Brünnhilde's Passionate Love leitmotiv intentionally to draw our attention to their shared themes? Could the theme have been inserted by Wagner unintentionally? Or could it be that Wagner simply wanted to use this marvellous theme again?
No written evidence or commentary by Wagner has been left to provide any answers. The mystery remains. Leitmotivs are woven into the fabric of Wagner's music in an elaborate and complex way that defies the simple labels commentators assign to them. The reason for the insertion of Brünnhilde's Passionate Love leitmotiv from Die Walküre into the finale of Tristan und Isolde will remain a tantalising mystery.