Cernunnos, the great horned one, was the archangel of Sucellus. He dared to attack Agares directly. Agares picked up the powerful archangel and showed him visions of himself dark and corrupted. So powerful was the vision that it became real, splitting Cernunnos into two creatures. They were identical in form but the new creature, which Agares named Hyborem, was dark in aspect and colored in blood. Hyborem and Cernunnos fought but neither could defeat the other. As such, even the archangel of Agares was stolen from another, he is a perversion of the natural...
The gods considered this breaking of the compact. They were unwilling to restart the Godswar, but couldn't let Mulcarn move into creation unchallenged. They agreed to let one god go into creation to fight Mulcarn. That god was Sucellus, the god of nature.
Sucellus entered creation and battled with Mulcarn. At the same time the Ljosalfar turned on themselves. They had agreed to shared power between two queens, one to rule during summer and the other during winter. The winter queen claimed dominance for years as winter showed no signs of ending and refused to give up the throne. This argument created a civil war and the new branch loyal to the winter queen called themselves the Svartalfar.
Sucellus lost the battle and was killed by Mulcarn. His body was broken in 7 pieces and scattered across Erebus. The Soul of Sucellus departed for the Underworld, where he helped Arawn, God of Life and Death, deal with the huge influx of souls that perished in the cataclysms that marked the beginning of the Age of Ice.
Creation froze even further and the elves were forced to stop their civil war as they could no longer mobilize armies or travel. They tried to find hidden places in the world to remain, and waited for the winter to end to continue their war. Facing first the brutal civil war and then the endless winter, life was extremely harsh on the Ljosalfar. It is unlikely that they would have survived, had not Cernunnos intervened. The greatest of the Archangels dwelt among them during this age, sheltering them as best as he could.
Once Mulcarn was vanquished, the seven pieces of the Sucellus' broken body were brought together in his tomb. Sucellus begged Arawn to let him return to life. To do this, he needed the Power of Life to be returned to creation. He also argued that this power was necessary if creation was to recover from the long winter. This was originally the Precept of Nemed, who had gifted it to Arawn when he chose to renounce his godhood and father the human race. Arawn abhorred the abuse of Life and (especially) Death magic that had occurred in the Age of Magic, and had sworn to himself never to return to creation, lest he again be used as a conduit that allowed dark sorcerers to create such abominations. He remained adamant in this resolve, and would not return with Sucellus no matter how much he was cajoled. He was convinced, however, to gift the Precept of Life to his fellow god, just as Nemed had to him. Sucellus took this precept and was resurrected, now as the God of Life. This fundamentally changed his personality, and he came to be seen by his fellow gods as a new Nemed, whom all the gods held in high regard. All the gods are honored to have him in their presence, and he alone among the good or neutral gods is welcomed as a guest even by Agares into Nyx itself, without warfare.
The changes in Sucellus were profound, as a god personality is the very embodiment of his precept. The resurrected Sucellus felt he could no longer fulfill his role as god of Nature (for Nature requires a balance of Life and Death, and his new preference for Life would have thrown the natural world out of equilibrium), and so he gifted his old precept to his first and greatest creation. Cernunnos left the Ljosalfar and ascended into the Vault of Nature as a god. Once the Greatest of the Archangels, he is now the weakest of the gods, for he alone was not made by the hand of The One.
Cernunnos is revered by the Fellowship of the Leaves, but not exactly worshiped. They worship Nature rather than its avatar, and have great reverence for Sucellus and Amathaon as well.