Basilisk Den is an intermediate grind-zone. Basilisk mobs are primarily grinded for money. The xp here is decent but nothing special. With that being said, this spot is decent, you can grind Polly's for better, more profitable rewards. Polly's is also one of the best spots in the game for xp.
Basilisk Den is located East of Altinova; Altinova, is located East of Heidel the second major city in Black Desert.