Brithvar can be encountered discussing with his lieutenant Kur at the duergar encampment in Grymforge at X: 624 Y: 355.
Like many duergar at Grymforge, Brithvar holds no loyalty towards the Absolute and resents Nere for withholding their promised payment. While initially suspicious of the party, as his ability as a duergar to detect Illithid presences makes him believe them to be a True Soul, he can be convinced otherwise and talked into leading a mutiny against Nere through advancing Blind the Absolute, causing half the duergar to fight against Nere once the drow is freed. After taking a long rest, he and the surviving duergar leave Grymforge before Absolute reinforcements arrive. He can be persuaded or intimidated into leaving the enslaved deep gnomes behind, allowing them to return to Baldur's Gate.