Per Sony Dev Jindrack (Infiltrating the Sanctum: Shadow Ordinance, post 14, 05/05/08), the Stone will drop in a chest from any Droag mob; killing sentinels, etc, will not produce the item.

--MerlinTKD 15:48, January 30, 2010 (UTC)

I slew serveral kazillion droag by now. Still no drop. Any ideas? Kennin 15:17, March 19, 2010 (UTC)

same here, any update on the drop rate? I have gotten chests, but none contained the stone. Mcook38 22:08, April 22, 2010 (UTC)

Hold fast. They're changing it to not require the trial stone, should be in by/with GU56. --lordebon 22:25, April 22, 2010 (UTC)

Thanks indeed! Kennin 10:57, May 18, 2010 (UTC)

great ty! Mcook38 18:29, April 23, 2010 (UTC)

According to update notes when you click the update notes button on the launcher, this was patched in a couple weeks ago:

Patch Notes May / 04 / 2010

SANCTUM OF THE SCALEBORN

The Trial of the Reaver no longer requires a Trial Stone to start. The statue that starts the event, "Rites of the Scaleborn," now becomes usable right after it spawns. Itpaladin 20:58, May 20, 2010 (UTC)