Well, you get it pretty easily if you ask me. Especially during Christmas its everywhere..i really really mean EVERYWHERE.
Its based on the Ragnarok Cycle of the Nordic mythology, especially the story of Baldur.
Wiki
Baldr is known primarily for the story of his death. His death is seen as the first in the chain of events which will ultimately lead to the destruction of the gods at Ragnarök. Baldr will be reborn in the new world, according to Völuspá.
He had a dream of his own death and his mother had the same dreams. Since dreams were usually prophetic, this depressed him, so his mother Frigg made every object on earth vow never to hurt Baldr. All objects made this vow except mistletoe. Frigg had thought it too unimportant and nonthreatening to bother asking it to make the vow (alternatively, it seemed too young to swear).
When Loki, the mischief-maker, heard of this, he made a magical spear from this plant (in some later versions, an arrow). He hurried to the place where the gods were indulging in their new pastime of hurling objects at Baldr, which would bounce off without harming him. Loki gave the spear to Baldr's brother, the blind god Höðr, who then inadvertently killed his brother with it (other versions suggest that Loki guided the arrow himself).
Its actually not a very fitting catch if you don't play a scion of Baldur, but after all the little mistletoe led to the events that caused the downfall of the Nordic Gods, thereby I deemed it a worthy catch for their off-springs.
And to be honest, when I created my Scion and his catch i was pretty new to the DF Game and thereby had some difficulties to come up with better catches, that weren't that worn and boring.
