Ooh, just got to the section in Cloak and Dagger about the Heralds and a possible decades long infiltration of Gargauth into their organisation.
From their Gargauths corruptors could make their way into the Harpers since the two organisations are very closely allied.
I did find it strange that the Heralds lost two High Heralds (Huntsman and Manyshields) when the holders of those posts became corrupted (presumably by Gargauth).
This corruption isnt unusual in itself but the fact that they havent been replaced is.
I had a random thought, that what if the holder of the office is merely a holder for some kind of vast heraldic database (like an elven kiira device), and the reason they havent been replaced is that the receptacle/database itself has been damaged or destroyed (with the death of the holder of that office).