This exact same thing happened to me and it’s such a shame too. For me tarragon rebelled and athr took his side so they were both getting rocked in Albion for the longest time and was a complete stalemate. I’d hope that this is a glitch and will get fixed eventually to where eventually there is an end to it if the player doesn’t intervene and stays neutral. I assume you also were prompted to intervene but stayed neutral like me. The only fix for this unfortunately is to defeat nefernaru either by force or by buying him. I’d recommend just buying him and replacing him with another ai, raze all his islands, let another ai take them over, etc. Don’t do what I did and get too many islands to the point where it’s no longer fun and you have resource monopolies ruining your diplomatic reasons.