Lake Liuan is also described by Geoffrey of Monmouth in his "History of the Kings of Britain" (~1136 CE). He relates a conversation between Arthur and a companion1 about the lake. A Welsh version of this conversation gives...
"And he told him also that there was another lake in Wales near the Severn, which the men of that country called Llyn Llivan; and that lake, when the sea flowed, received water into it, and swallowed it as though it had been a mountain, until it overflowed its banks; and if it chanced that any stood with their faces towards the lake, and any of the spray of the water touched their clothes, it was hard for them to avoid being drawn into the lake; but if their backs were towards it, how near soever they might stand to its edge, it would have no effect upon them."2
This might be a mistranslation of the version by Nennius, or it may suggests the water itself has magical powers, drawing in bystanders to their doom. While the lake isn't noted for its special character in the tale "Culhwch and Olwen", it does suggest the lake is also the home3 of the oldest animal in Britain - a giant Salmon4, who helps Arthur's men rescue a prisoner from upstream of the lake at Gloucester as part of their quest for the giant boar Twrch Trwyth. This quest also features in the Wonder of Builth Cairn. You can read more about "Culhwch and Olwen" and the hunt for Twrch Trwyth Here.
However, if the Wonder is associated with the River Troggy, there is a second mythical pig-hunt associated with it: that of the pig Henwen: More.