Baldr
When Loki learned of a prophecy entailing the death of Baldr he leaked the information knowing that Frigg, wife of Odin and mother of Baldr would do anything to protect him. Frigg proceeded to extract an oath from all things living and not that they would not harm Baldr making him in essence invincible. However Frigg felt the new plant mistletoe was too young and unthreatening to take the oath.
Proceeding with his plans Loki took a twig of mistletoe and formed into an arrow. As Baldr’s step-brothers passed the time by striking at Baldr with various weapons Loki approached the blind god Hoor and bade him take a turn. Giving him the arrow of mistletoe he guided the shaft unto Baldr’s heart.
For his part in Baldr’s death Loki was bound in a dark place where a serpent would drip burning venom on his head until the end of time, but Loki knew that Baldr now belonged to his daughter Hel who belonged to him, he knew that when the time comes Hel will have remade Baldr’s mind and he will have an invincible sentinel on the day of Ragnarok.