For the Valdemar setting, Mercedes Lackey created the lifebond, a rare type of relationship where two souls become linked. It is not something that occurs by willpower, and no amount of magic can make it happen. Though many yearn for such a joining, the reality of it is much different than they realize. Poets and Bards make it sound like the epitomy of love, when it is about compatibility. Love nurtures the lifebond, but it is not part of the bond itself. Emotions resonate along the bond, so if one person is unhappy, the other will know it. It doesn't prevent misunderstandings and arguments, but the pair is forced to deal with issues promptly or suffer from reflected turmoil.
As one character in Winds of Fury notes about when a lifebond will happen, "...it is likelier that someone with an extremely powerful Gift of some kind and a tendency to deep depression will be lifebonded than someone who is not so burdened and hag-ridden. That's so the Gifted-and-suicidal half has someone outside of himself to keep him stable and give him external focus. But--all we know is that while it's rare, it isn't something to be yearned after."
Lifebonds are a frequent theme in the series. They occur at a higher rate among Heralds than the general populace. The Arrows trilogy has a couple pairings. Mage Winds and Mage Storms trilogies have characters yearning and/or actively seeking a pairing. Characters discuss how and when they occur. And there are legends about famous pairings.
One of the most famous lifebondings in Valdemaran lore is the story of Lythe Shadowdancer and Rothas Sunsinger. Fate rested heavily on these two lovers. Twilight was the only time they could be together. Lackey has specifically stated she has no plans to write their story in full, preferring to leave in legend, but there are a few songs about them, a whole CD now, called Sun and Shadow. But before that, I was tantalized by the mention of them in the Arrows books and a couple songs in the accompanying Arrows music collection, plus one in the Last Herald Mage music collection.
After they met, they faced many trials to gather the magic they needed to break the curses keeping them apart. But once they finally had the power, they faced the hardest trial yet: use the power for themselves or spend it to save two others. With pain in their hearts, they selflessly chose to free Windrider and Darshae from the clutches of a Dark magician. This act led to them being Chosen as Heralds, which later led to their eventual freedom and joy.
Here are the songs of their meeting and of the plight of Windrider and Darshae. The heartache grips me every time. Kristoph Klover (as Herald Kris) sings of their meeting, while Heather Alexander sings of their sacrifice.
If you enjoy these songs, I recommend picking up the CD.