Later, Odin had another son named Balder. Loki never liked him because the god of light and truth (Balder) was the most handsome of all gods. Balder was also knowledgeable in healing herbs and runes, which made him a favorite among the people of Midgard. Balder lived in a palace named Breidablik with his wife Nanna (n.b. there is also a Mesopotamian goddess of this name), a vegetation goddess. It was believed that no lie could pass through the walls of Breidablik, home of the god of truth, so when Balder started having frightening nightmares about his own demise, the other Aesir gods took them seriously. Unlike gods in other pantheons, the were not immortal. They cataloged everything that might possibly cause Balder harm, from weapons to diseases to creatures. With the list in hand, Balder's mother, Frigg, set out to exact assurances from everything in the nine worlds not to harm Balder.
When she had completed her mission, Frigg returned to Gladsheim, the gods' meeting hall, for a celebration. After a few rounds of drinks and toasts, the gods decided to test Balder's invulnerability. A pebble thrown at Balder bounced off without hurting Balder, in honor of its oath. Larger weapons were used, including Thor's axes and all refused to hurt the god.
Loki is known as a trickster god. Sometimes he was mischievous, but he hadn't really been malicious. The giants were evil, but Loki, who was the son of a giant, hadn't been known as such. It seems his self-appointed job was to stir things up when things were going well. It's a Loki-type action that one wishes to avert when telling an actor to break a leg before a performance.
Loki was disturbed by all the gaiety and decided to do something about it, so in disguise as a disgusting old hag, he went to Frigg while she was at Fensalir taking a break from the festivities. What was going on at Gladsheim, he asked her. She said it was a celebration of the god Balder. Loki-in-disguise asked why, then, were people throwing weapons at him? Frigg explained about the promises she'd exacted. Loki kept at her asking questions until she finally revealed that there was one thing she hadn't asked because she thought it too small and inconsequential. That one thing was mistletoe.
With all the information he needed, Loki set off to the forest to get himself a branch of mistletoe. He then returned to the festivities at Gladsheim and sought out Balder's blind brother, Hod, god of darkness, who was in a corner because he couldn't aim and therefore couldn't participate in the test of Balder's invulnerability. Loki told Hod he would help him take aim and handed Hod a piece of apparently innocuous mistletoe to throw.
Hodur was grateful and accepted the offer, so Loki steered Hod's arm. Hod launched the branch, which caught Balder in the chest. Balder died instantly. The gods looked towards Hod and saw Loki beside him. Before they could do anything, Loki fled away.
Celebration turned to lamentation since the most beloved of the gods had died. Odin alone was aware of how disastrous this event really was for them all, for he knew that with the loss of light and truth, the end of the world, Ragnarok, was due soon.
A funeral pyre was made that was so enormous the gods had to ask the help of the giants. They then placed their most valuable worldly possessions as gifts upon the pyre. Odin placed his golden armband Draupnir. Balder's wife fell down dead of grief at the pyre, so her body was placed beside her husband's.
[After their funeral, they were in the world called Niflheim.]
An attempt was made to resurrect Balder, but due to more of Loki's mischief, it failed.
The goddess of death, Hel, promised that Balder could return to earth if every living creature shed tears of grief for Balder. It looked as though it would work, for everyone loved Balder, but Loki arranged for a single exception. Loki disguised himself as the giantess Thok. As Thok, Loki was too indifferent to cry. And so, Balder could not return to the land of the living. Balder and his wife remained in Niflheim.
Another son of Odin, Vali, revenged the death of Balder, but not by getting back at Loki. Instead, Vali slew his brother, the blind god Hod. Loki, who had fled the initial scene of Balder's death in Gladhseim, and then re-appeared in disguise as the giantess Thok, tried to get to safety by turning into a salmon. The salmon-Loki hid in a waterfall. But the Aesir, who knew where he was, tried to catch him in a net. Loki was too clever for that and jumped right over the net. Thor, however, was fast enough to catch the leaping fish in his bare hands. Then Loki was bound in a cave with venom dripping onto his body, which caused him to writhe in pain.