The Northeast Pillar is a pillar in the Frozen Reaches near the Impervious Ice that explains in undecipherable text how to get the Flame Sword.
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The pillar can only be translated through use of the Deciphering Amulet. It reads "combine the Jewel of Fire with icy water to produce magical steam to melt the Impervious Ice."
After getting the Deciphering Amulet from the Ice Lord's Stronghold after killing Thork, Connor was able to use it to read the pillar, melt the ice and get the Flame Sword which he used to kill the Two-Headed Dragon to continue his journey.