Loki was never really well-liked by his own people, the Jotunn, the frost giants. It was sometimes said he was only part Jotunn, and the frost giants did not like half-breeds. Loki responded to this rejection variously. Sometimes he sought to be impressive, to provide solutions for the problems of his people. Other times, his frustration got the best of him, and he sowed misfortune upon his people using his great cunning to exact revenge unto his tormentors.
Loki and Odin met while they were each seeking magic, and they found in each other something they each wished for terribly. Odin saw in Loki, who was a shapeshifter, a most magnificent puzzle he longed to solve. And Loki saw in Odin the warm acceptance of a truly wise father. They traveled together and had many great adventures and discovered many secrets. Loki asked that they be bonded as blood-brothers in a magical ritual. Odin, agreed and vowed even to never accept a drink unless drinks were offered to them both.
After Odin married the lady Frigg, and they conceived and raised up the first of the Aesir, the thunder god Thor, Odin and Loki drifted somewhat apart. Odin busied himself with fathering more children, making war on their neighboring tribe, the Vanir, and welcoming those that wished to join the Aesir, noteably the beautiful twins, Freyr and his twin sister, Freyja, who became gods of fertility and beauty. Another child of Odin, Baldur, was Frigg's greatest pride, and became the god of justice, purity, peace, light, and gaiety. Odin also worked on raising Valhalla, the sacred hall where heroes who fall in the midst of valor in battle are taken to gather, drink, sing and tell tales, and joyously await the final battle.
As Odin's family grew, Loki's jealousy grew, and his attempts to impress Odin and be otherwise included in Odin's family, became more extravagant. He went off to study the runes as Odin had, and learned complex magicks, and explored the dream world. He went off to make deals with the dwarves for their magnificent treasures for Odin, acquiring through trickery a magical golden ring which made duplicates of itself, and the legendary hammer Moljnir, which Odin presented to Thor. He crafted interesting objects, and invented tools, including the fishing net, which he gave to mankind. Loki even thought to have children of his own, and, because he wanted to be so deeply involved in his children's life, he chose to give birth to them himself.
In perhaps his most interesting adventure, Loki was called in to help solve a problem the Aesir were having with a giant who was building a great fortification for Valhalla. The gods had agreed to pay the giant a tremendous unreasonable fee, including the goddess Freyja, under the condition that he complete his works before summer, which the gods thought near impossible. But the builder had a fabulously strong stallion helping him with the project and his work was much faster than expected. Loki cleverly took on the form of a beautiful, alluring mare, and caused the stallion to give chase for long enough that the builder's deal was forfeit. Some time later, Loki came to mother a stunning, eight-legged flying horse, whom he named Sleipnir and presented to Odin, who was overjoyed and from then on always rode Sleipnir into battle.
The young gods were not as appreciative as Odin was of Loki, and thought that maybe his giving birth to an eight-legged flying horse was pretty strange. They were often confused by Loki's ways, in fact, and often showed him much mistrust and disrespect. They did not truly accept him, and sometimes went so far as to conspire to leave him out of their gatherings.
One such gathering was precipitated by a prophetic dream. Baldur worried to his mother that he had dreamed of his own death, and that he was afraid. Frigg was worried too and took drastic measures. She went on a quest and asked all of the elemental forces of the world, the trees, the plants, the rocks, the flames, the waters, all of these for their solemn vow to never harm her son.
She came back from the quest feeling victorious, she called out to her court, "Glorious Aesir, my child, Baldur, our god of joy is saved, he is invulnerable to all harm, I have made it so. Let us celebrate!"
So they had a big party, and Frigg invited 13 of the gods, Odin, Odin's sons, grandsons and adopted sons. And all 14 of the goddesses, including Frigg herself. And there was much drinking and merrymaking and excitement. And the gods riled themselves and one asked "So Baldur is invincible, right? Then let us test the matter!" And so the manly gods of the Aesir began hurling spears and firing arrows into Baldur; and he merely laughed and the arrows passed near him, or bounced off him, or Baldur was inspired to catch them right out of the air. And all laughed and drank and became ever more rowdy.
Loki was not invited to the party. He was the only of the gods not invited, in fact. Even blind Hodur was there, standing around, awkwardly.
But Loki heard about it, oh indeed he did.
He heard that all of Odin's family was attending a marvelous, exciting event, and he could hear their laughter and knew their joy, and he was filled with longing, and sadness, and then, gradually, a terrible fury.
So he shapechanged into a serving girl, and went to the celebration and took in the sights and saw his beloved Odin, there, drinking without him. And his fury turned to terrible purpose. He went up to a merrymaker and asked, all wide-eyed and innocent, "If you'll pardon a simple girl's curiosity, why is there all this uproar?"
"The Lady Frigg, praise be to her name, has rendered Baldur invincible! Her quest was a success! See how all objects are as nothing to harm him!?"
So Loki went to the Lady Frigg and offered her more drinks, and flattered her, "My grand lady, I have heard some speak of your quest. You sought all the beings of the realms to swear to you not to harm your son. That is amazing! You astonish me!"
Frigg, who was deep in her cups, replied "I know. I'm awesome."
Loki then prompted "It sounds like your quest must have been rife with hardship. Were not some of your subjects stubborn, challenging to you?"
Frigg thought about it for a tipsy moment, then said "Some of them were kinda stupid. I had to use hand gestures and such. And there is a plant in the deep forest named Mistletoe too new to the realms to understand such oaths. But she is surely harmless. A cutie-pahtooty she was. I wanted to just kiss her."
Loki took his leave and vanished from the party, heading to the deep wood. When he came upon mistletoe, he asked grandly "Mistletoe, I have spoken with the queen of the Aesir, and she has told me that you made no such vow as all the other creatures made. Is this true?"
Mistletoe responded, sweetly "Oh, you are cute. Would you kiss me?"
Loki concluded that Mistletoe was too young to understand vows and such, and took a piece of her away with him and forged a magical arrow from it. Then, disguised again, went back to the party, and went to Baldur's brother, Hodur. And asked why he was not participating in the fun.
Hodur says "Well, you see I am blind, so I would not be able to strike the mark. And for related reasons, I have no weapon."
Loki said, full of cunning, "That seems like no reason to keep from honoring your mother's celebration and showing the All-father that all of his son's are powerful! Here, you may use my bow, and my arrow, and I will guide your hand to the mark."
Hodur was so convinced and all gathered to watch the blind man fire an arrow into his invincible brother.
The magical arrow, fired by Hodor, guided by Loki, struck true, and passed directly through Baldur's heart. And Baldur was instantly killed.
A pall descended upon the gathering. The god of joy had died in the midst of celebrating his immunity from that very fate. Most began to weep. Some cried out to Frigg, "Goddess? Your plan? How could this have happened?"
Frigg was rife with shock and babbled that she did not understand.
When the shock was wearing off and grief began to take its place, Loki emerged in full glory, wreathed in flames. His eyes glowing with terrible fury and worse disdain. Instantly, all knew that he was the cause of this calamity. He approached Odin, and stared him down in betrayal.
Finally, he spoke, his eyes narrow and cruel...
"Next time, just invite me to the friggin' party."
And that is the story of how Loki crashed the party, which had 13 gods attending, making the number 14, dedicated to Frigg, the queen of the Norse gods, for whom Friday is named. The day joy died making the number at the party once again 13. Friday the 13th has therefore always been a reminder of this terrible day, and unlucky.
Baldur was also the god of forgiveness, and now dead.
See that wolf? that is one of Loki's children, enspelled to go all berserker on his brother. See what they're binding Loki with? Yeah, those are entrails, of so berserked upon son. See that snake up there on the right? Yeah Loki and that snake have an AWFUL lot of good times together.