To enter the Dargean Inner Sanctum within Horn Peak Mountain, one must first recite the series of spell glyphs from each of the FIVE types of element stone...
Fire
Ice
Air
Earth
Death
The party have visited and received the series of spell glyphs from the Air Stone located in the Faunar Watchtower.
They have activated the corresponding seal on The Door Into Horn Peak Mountain.
The party have visited and received the series of spell glyphs from the Death Stone located in the Desecrated Iron Mine.
The party have visited and received the series of spell glyphs from the Earth Stone located in the Emerald Peaks.
The party have visited and received the series of spell glyphs from the Ice Stone located on The Frozen Isle in the Hourglass Sea.
The party have a lead that a fire stone is located in the Desert of Thorns.