Utgard-Loki was a powerful giant from Norse mythology and an opponent of the Norse gods, he is famous for his cunning and magic - which he used to torment Thor and Loki to test the might of the gods without their knowledge, as a result of Thor's deeds during these tests Utgard-Loki came to the conclusion the gods were too powerful to safely battle and thus made a hasty escape while decreeing he never wished to make contact with the gods again.
He is one of the few opponents in the Norse myths who not only challenged the gods but outwitted them and lived to tell the tale (indeed he is not even punished, simply fleeing due to becoming worried on how powerful his opponents were and thus deciding wagging actual war against them would led to his destruction).
History
Utgard-Loki wished to test the strengths of the gods but was much more intelligent than most giants, deciding to use powerful magic and trickery rather than brute strength - in doing so he was able to achieve considerable victories over Thor and Loki but grew increasingly fearful as he watched them apparently "fail" at each task given, only to reveal how even their failures were feats of immense strength.
For example he had Thor drink from a horn, while Thor could only drink three large gulps before giving up he had in reality been drinking the ocean itself and had drunk so much of the ocean that he formed the tides.
He also had Thor lift a cat, yet the god could only achieve lifting a single paw of the feline, again this secretly terrified Utgard-Loki as Thor had in reality been lifting the Midgard Serpent - a snake so vast it was said to circle the entire world.
Finally he had Thor wrestle a servant, whom Thor ultimately lost to despite a great struggle - this was revealed as Thor fighting Old Age itself, the god only being defeated because none (not even the aesir) can truly defeat Old Age.
After observing these and other feats of strength by Thor and Loki Utgard-Loki revealed himself and told them of what they had achieved, furious at being tricked Thor tried to use his hammer to kill Utgard-Loki but the giant disappeared and thus evaded the thunder-god's wrath, though his interaction with the two gods left such an impression upon him that he decided not to trouble them again.
