The dark idol is a Dark Maker artifact seen in Jak 3. It is presumably a corrupted Precursor idol. This helped Jak use the Dark Invisibility dark power, which allowed Jak to bypass the sentries and other defenses guarding the sanctions of Monk Temple after touching it. Dark idols normally would have petrified its subject but for Jak it translated into power due to his dark eco injections.
One dark idol was apparently found next to a dark satellite that had crashed within the Great Volcano. It was eventually discovered by a Precursor Monk expedition and simultaneously led to one of their member's demise.