I think somewhere in the books it was mentioned that Yarth were posing as good guys, so I will explain how I did it in my game.
Around 6020 some clerics appear claiming to be the religion of of the Sea-Drake (the old U-Lyshak) and using the Sea-Drake emblem, they teach altruism, be fair, later they opened a few hospitals in Cynar and Norek to attend the sick, and they claim to be the guardians of the old religion opposed to other religions like Orhan that is mainly coming from Rhakhaan.
In 6038 the High Cleric takes residence in the old tower of Yarth, a well know power place for clerics since millenia. Loremasters and Orhan clerics suspect that something is wrong, but everyone accusing them is attacked as being paid by Rhakhaan.
In my game, the hunting accident of Kier's father was while hunting whales in the Bay, that was used by the priests to explain that the sea has called him because they were deviating of the Sea Drake and worshipping Orhan gods.
So with a mix of "good old days" when we were an empire as big as Rhakhaan, and a profile of healers and advisors, plus lots of gold and the fear of having people like Lord Boskhar of Vorn pushing to be the regent, a clerical figure was more convincing to the nobles of Helyssa.