"Its old master served my patron well."
It's a clear quote that means there is a leader of Shadow Isles, not like next quote.
"A crown for the king of death."
He can be used it for himself but I don't think so.
"You are but the herald, Hecarim. Remember who you truly serve."
Many can say Hecarim serves to Shadow Isles but Shadow Isles is a not person, a place. You can't call it as '' who ''.
I think he's just talking about a personification of Death in all those quotes. And remember, Kindred is only one personification of death in Runeterra. There are possibly many more. Especially for the undead.
"Its old master served my patron well."
This is quite clearly talking about Rabadon and probably how he caused the end of many lives.
"A crown for the king of death."
Appears to be just a generic title he is bestowing on whoever is wearing RDC (in this case himself and probably Rabadon). Note this quote would be much different if it read, "A crown for the King of Death". As then he would be talking about a specific entity.
"You are but the herald, Hecarim. Remember who you truly serve."
Again, this is Death. Hecarim is the herald of death. Karthus is reminding him that he is a servant to a greater cause, not his own master.
<-- also this
Shadow Isles isn't really a place, based on all the lore, it became an entity of death, so it MAY be possible that they are reffering to the island as an entity, instead of a single person.
"Its old master served my patron well." I guess that the "patron" of Karthus is supposed to be Kindred, even if they do not actually support those "ideologies", in fact, Kindred is against them, but Karthus thinks that he is helping them.
"A crown for the king of death." Well, there are 2 "kings of death", Mordekaiser and the Ruined King.
_ "You are but the herald, Hecarim. Remember who you truly serve."_ Hecarim served the Ruined King before the "accident" of the Blessed Isles... that is why it says "Remember who you truly serve",
The shadow isles is a mystery in it of its own.
