Siegfried bathed in Fafnirs (a dragon) blood, thus becoming invincible whereever the blood touched his body, but...
A treacherous leaf
And a secret unsealed
It will cause deadly wounds
a leaf fell on his shoulder unnoticed. The blood didn't touch him there, so this was his achilles' heel.
Later on, he is betrayed by his loved one, who knew about the spot on his shoulder. She is convinced by Hagen of Tronje to "mark the spot so I may watch over it in battle." And so on.
This is clearly about the Nibelungs.
Siegfried bathed in Fafnirs (a dragon) blood, thus becoming invincible whereever the blood touched his body, but...
A treacherous leaf And a secret unsealed It will cause deadly wounds
a leaf fell on his shoulder unnoticed. The blood didn't touch him there, so this was his achilles' heel. Later on, he is betrayed by his loved one, who knew about the spot on his shoulder. She is convinced by Hagen of Tronje to "mark the spot so I may watch over it in battle." And so on.