Aegir is the Norse god of the sea. He has dominion over the oceans and the waves, and throws great parties for the gods. His secondary domains of divine influence are brewing and hospitality.
Most likely bearded, Aegir was probably shown as a well-muscled middle-aged man. He was probably tall, since his father Fornjot was a Giant. It’s possible he was crowned with seaweed. He likely would have been shown around or in water, or with his children, but no other attributes are given for Aegir.
I couldn’t find any specific myths in which we is the primary character, but he is mentioned sidelong in other myths, and this is how we know of him. He appears in Snorri Sturrluson’s Edda, the most complete collection of Norse myths available.
What is known about Aegir is that his father was a Giant. His siblings include Logi, the fire Giant, and Kari, the wind Giant. Although they’re called Giants, these figures are also deities. Aegir’s wife was Ran, goddess of the sea. They had nine daughters, all different kinds of waves, such as pitching wave, surging wave, blood-red wave, etc. (they did have real names, of course: Bara, Blodughadda, Bylgja, Dufa, Hefring, Himinglaeva, Hronn, Kolga, Unnr). Aegir and his family lived under the sea on or near an island called Hlesey.
Aegir once threw a great party for the gods, for which he and his daughters brewed the ale. The enormous cauldron was provided by Thor, and there is a whole story about how Thor retrieved this special cauldron. This tradition of Aegir throwing a party every year is where his secondary domains of hospitality and brewing come from.
Light and Dark Sides
On the positive side, Aegir seems like a really cool guy. He throws the gods an awesome party, volunteering to make the ale for them, and runs his oceans just fun. All in all, he seems pretty nice. Without other myths to go by, there isn’t any proof for a Dark side, but we can speculate on what his Dark side would be like. Considering that the names of most of his daughters are names of dangerous waves (chilling, surging, grasping, etc.) I think we can conclude that Aegir’s Dark side is expressed through his children–the oceans can be dangerous places that mankind could have trouble with and definitely would die in. Oceans can be wild, uncontrollable, and unpredictable, and I think that is Aegir’s Dark side–the natural counterpart to his calm, loving, laughable Light side.