This motive accompanies the lines, “So should we die, no more to part, ever one in endless joy.”  Scruton claims that this is a variation of Longing due to the descending line of three half steps.

Motive Labels
“So stürben wir…” – (Fischer, p. 14) and (Redlich, p. 26)
“Liebestod” or “Love-Death” – (Kobbé, p. 127)
“Liebestod, Unendlichkeit” or “Love Death, Infinity” – (Pfohl, p. 215)
“Song of Death” – (Lavignac, p. 304)
“Scheidegesang (auch Motiv von Isoldes Liebestod)” or “Parting Song (also a motive of Isolde’s Love-death)” – (Windsperger, p. 44)

Liebestod – Order of occurrence by Act and Scene
Act II: Scenes 2-3
Act III: Scenes 2-3