This item is dropped off by only the iksar behemoth golem. It's PH is a massive golem. I have hunted this golem many times and have got nothing from the massive golem only from the behemoth.
The Iksar Behemoth spawns in the large open area populated by "an iksar golem". This area is visible through the barracades from the first large open room from the entrance, but requires a bit of a crawl to work one's way around to.