Golem Essences are materials dropped by the River Golem in the Epic Boss/Summoning Stone area.
After you get 50 of them you can craft the legendary armor Riversteel Robes.
After you defeated the Riveer Golem 15 times in the Proving Grounds you can craft the first armor.
Defeating it at level 43 you can craft the second one. Once you defeated it at level 60 you can craft the third armor.
You get 1 Golem Essence defeating it from level 2 to level 20,
2 Golem Essences by defeating it at level 1 and from level 21 to level 46,
3 Golem Essences from level 47 to level 59 and
4 Golem Essences since level 60