In The High King they slew the high-king Math and in The High King, during the battle of Caer Dathyl, they caused the attacking army to draw back into formation, parting like a gate as the Cauldron-Born marched towards the castle.
In the first book in the series, Arawn sent his general, the evil Horned King, to amass a vast army with which to topple Caer Dathyl, the mightiest city in Prydain.