The passage alludes to slavery in its various forms.
The first two sentences recall colonial slavery, where inequity between men was cemented in law ("even the five largest gems in the world were worth no more than one of the dark tribunals") and darker-skinned peoples were forced into agricultural toil ("with whip and terror they forced the vast darkness into the sun").
Slavery still exists in many forms today (and in numbers vastly greater than at the time of European colonialism). Nearly 40 million people are classified as slaves, which doesn't include de facto types of slavery such as sweatshops and towns under military occupation. The type of slavery most referenced in the mass media is sex slavery and its many variants, and is also suggested by the use of the word "lust" following the "greed" of the colonial slavers.
The final three sentences in the first paragraph suggest a willful silence or neglect ("they skulk and laugh at their own invisibility") concerning the modern practices of slavery. In some cases this arises due to a general lack of awareness of the problem, in some cases due to apathy, and in some cases due to the disconcerting similarities between modern slavery and colonial slavery ("those who were defeated live on in the tears that nobody cares to wipe away").
A "dark tribunal" is therefore a reference to a slave owner or slave trader, and the crime being ignored is the modern practice of slavery.
"Tribunal" is a curious choice of word. It literally refers to a seat or court of justice, and when applied to men refers to a plurality of judges or court officers presiding over a matter of law. "Dark" in "dark tribunal" presumably refers to the darkness in the practice of slavery. There may be some deeper symbolism here, but I've rambled on long enough already. ;)