Baldr

Baldr is a character in Norse mythology who is believed to be the son of the god Odin and the goddess Frigg. He's known as the god of light, joy, beauty, and purity.

Now, the story goes that Baldr had a dream where he saw himself dying. His mother, Frigg, worried about her son's safety, goes around to make sure that every single thing on earth promises not to harm her son, Baldr. She goes to every creature and thing, including plants, animals, and even stones and asks them if they would promise not to harm her son. They all agree, but Frigg thought that the mistletoe, a plant that grows on trees may not cause harm to Baldr and so didn't ask it during her mission.

Loki, the god of mischief and jealousy, had a problem with Baldr's popularity and decided to plot against him. Loki crafted a dart made from mistletoe and gave it to another god, Hǫðr. When Baldr was playing and everyone was throwing things at him to see if they would harm him, Hǫðr threw the mistletoe dart that Loki had made at Baldr, killing him instantly.

The death of Baldr brought great sadness to the gods and goddesses and eventually, Odin, Baldr's father, decided to bring him back to life. Before doing that, he asked for a favor from the goddess of the underworld, Hel. Hel agreed to release Baldr if all things wept for him. All things agreed except for a giantess named Thokk, who was actually Loki in disguise. Because of her refusal to weep, Baldr was unable to be brought back to life.

So, in summary, Baldr is a god of light, joy, beauty, and purity in Norse mythology who was killed by a dart made from mistletoe, which wasn't included in the promise of not causing any harm, fulfilling his dream completely.