What is the meaning of "his downfall was a mistletoe"?

The song is about Ragnarok from Norse mythology, so it's that story that you need to research to find the meanings in the lyrics.

The wily and disloyal Loki sensed an opportunity for mischief.

In disguise, he went to Frigg and asked her, “Did all things swear oaths to spare Baldur from harm?” “Oh, yes,” the goddess replied, “everything except the mistletoe. But the mistletoe is so small and innocent a thing that I felt it superfluous to ask it for an oath. What harm could it do to my son?” Immediately upon hearing this, Loki departed, located the mistletoe, carved a spear out of it, and brought it to where the gods were playing their new favorite game.

He approached the blind god Hodr (Old Norse Höðr, “Slayer”) and said, “You must feel quite left out, having to sit back here away from the merriment, not being given a chance to show Baldur the honor of proving his invincibility.” The blind god concurred. “Here,” said Loki, handing him the shaft of mistletoe. “I will point your hand in the direction where Baldur stands, and you throw this branch at him.” So Hod threw the mistletoe. It pierced the god straight through, and he fell down dead on the spot.

I have bolded the important parts here.

(Source - https://norse-mythology.org/tales/the-death-of-baldur/)

Not knowing the lyric/song, the phrase might have been taken as the sum of the words.

His downfall was a mistletoe

• His: there is a male involved, who is the subject of the sentence
• Downfall: Something has happened to decrease the status of the subject
• was: past tense, so it has already happened
• a mistletoe: Singular - one piece of mistletoe was the cause.

According to https://en.wikipedia.org/wiki/Mistletoe the plant is toxic but not normally fatal.

In pre-christian era, mistletoe is a symbol of fertility and virility.

In western culture mistletoe is associated with Christmas as a decoration under which lovers are expected to kiss. It may be hung from the rafters or ceiling or perhaps in an entranceway to allow people to stand underneath.

Put all that together and one might get

A male suffered after an event in late December. The problem may have involved a romantic companion, and/or a pregnancy.