I picked this up because I got the second one in the series through Goodreads' First Reads program, and figured I should start at the beginning.
Apparently, Grimbert is a popular, prize-winning author in France - this is his first book to be translated into English. There are, at this time, no fewer than 13 books in this series.
'Six Heirs' is basically an introduction to the characters and the scenario. Long, long ago, a mysterious emissary invited representatives of local kingdoms to a meeting on a small island. The representatives - those that returned - never spoke of what transpired during that meeting, but they may have passed their secrets down to their descendants. A tradition has sprung up for the descendants to meet at the island every three years for what has become something like a festival and family reunion.
However, now, someone seems to have hired the sinister assassin-priests of Zuia to wipe out every 'heir.' As you might guess from the title, six of these random people get together and try to figure out what's happening to their friends and family, and why. We've got: the lovelorn village boy, Yan. The object of his affection, Leti. Her guardian, the older wise woman, Corenn. A peaceful giant, Bowbaq; a warrior: Grigan - and finally, the handsome and roguish actor, Rey.
Things unfold in a manner in concordance with the standards of the fantasy genre. It ends on a bit of a cliffhanger - which makes me glad I've got the second book ready-to-read.