Finished my lvl 80 final destiny quest. Very enjoyable questline. You talk to Kalanthes once more and he sends you on a few errands that end you up having a special charm made to enter the Onyx Chambers w/o drawing aggro. So you walk the chambers until you find your way to a crack in the wall where you can sneak into Thoth-Amon's temple unscathed; this crack is on the right side of Onyx as far down as you can go (there's a shaft you must jump down at some point). Make sure you refresh your charm so as to not draw aggro.

Once in the crack you make your way to a HUGE antichamber loaded with packs of noid mobs. Kill each pack separately until you clear it out. Once clear you open sarcophagus' of friendly gods that will give you super buffs; these buffs literally turn you heroic. Then you dismantle the tablets holding the Grim Grey God at bay one by one. After each tablet is dismantled a group of mobs will spawn and try to take you out.

The best way to proceed is to range pull the mobs once they stop and hope they split off. Then take them out. If you can make it back to the sarcophogus' you can stack the buffs to make you even more impermeable. After each group is killed a new sarcophagus becomes openable, thereby giving you a new kind of buff for the next set of mobs that spawn after you break the next tablet.

Rinse, wash, repeat.

Each breakage will piss the God off more and more and he'll spit uber waves of godlike energy down the chamber. Avoid this stuff at all costs. Never fight the mobs in the way of this energy. Make sure you engage them to the sides where there's cover. Again, stacking the buffs at the sarcophogus' is a good way to get through it.

The final sarcophagus contains a pissed off zombie boss that u must kill (i think; could have it in the wrong order). Once this is done you can dismantle the final tablet and end the God's existence.

Report back to Conan for your epic reward.

Here are some pics: