When you speak to Vaska (#1) in the Swamp Forest she’ll tell you about the Golem acting as a Sentry to the Mage’s Tower. Somehow you’ll need to awaken the Golem and defeat it to find one of the last Sephirah.
Vaska will give you the Tower Tarot Card which you can take back to show Kalkstein in the Temple Quarter. Kalkstein will figure out how to wake up the Sentry and also give you the Golem’s Pith formula. To wake up the Sentry you’ll need to make a Lightning Rod. You can make it by visiting one of the Blacksmiths. Either the Dwarven Blacksmith or the Order Armorer. Both will make the Lightning Rod for between 50 to 70 Orens.
The Lightning Rod isn’t enough to wake up the Golem, you’ll also need a thunderstorm. If it isn’t already stormy in the Swamp Forest then go over to the Druid’s Grove (#2) and speak with the Elder Druid. He’ll agree to make a storm for 500 Orens. He also tells you about the stone posts next to the Golem that will activate the lightning.
Make your way over to the Golem (#3) and click on it. Place the Lightning Rod on the Golem. It will get a shock of lightning and wake up. You can kill it slowly with the steel sword and using signs but it takes a long time. A faster approach is to activate the three pylons while the Golem is in the center. It will take a hit of lighting and lose about 1/3rd of it’s health.
Once the Golem has been killed search the remains for the Golem’s Obsidian Heart and the Neh’tza Sephirah.