There is no real quest for the holy avenger.
You get it after you kill the dragon Firkraag in the Windspear Hills.
Complete the map, kill the mean red dragon and one of the finest weopons in the game is yours !!
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BTW, this is a tough fight, unless you are at a reasonably high level be prepared for a few reloads. It is one of the better fights in the game though-and well worth the time and effort.
Good Luck!