Abac is the Irish form of the Celtic dwarf Addanc, a marine monster which dwelt by lake Llyon Llion, the "Lake of Waves" and caused a flood. Another alternative spelling is Avanc.
The story of Addanc is part of the Celtic Creation Legend. His end is attributed to Hu Gadarn, an early Celtic culture hero, who dragged the creature from its lair using his oxen. Hu Gadarn is ancestor to Cymry.
Another myth from the Mabinogion attributes Addanc's destruction to Peredur, a knight of Welsh myth who was educated by the twelve witches of Gloucester, a sign of his royal blood. Using a stone which rendered him invisible, he was able to slay Addanc. In the story of Owen and in the story of the search for the Grail, his name becomes Percival.
Dwyvan and Dwyvach escaped the deluge in a ship built by Nefyed Nav Nevion. Though this story is pre- Christian, like other Flood stories, it has suffered Christian additions.