You should obtain 15 Roentgeniums of Nulgath before doing the Void Crystals because you can earn reagents for the Void Crystals as you get Roentgeniums - it's more efficient.
To start, enter /tercessuinotlim and go to the Void Highlord NPC. If you've never been to /tercessuinotlim before, use the guide. After, accept the "Void Highlord's Challenge" quest from the NPC. If you can't accept it, look at the Requirements tab above.
Use the Reagent Library for details on how to get the following items (the total required amounts are parenthesised):
Aelita's Emerald (15)
ArchFiend's Favour x300 (4500)
Black Knight Orb (15)
Elders' Blood (17)
Emblem of Nulgath x20 (300)
Essence of Nulgath x50 (750)
Gem of Nulgath x20 (450)
Nulgath's Approval x300 (4500)
Nulgath Shaped Chocolate (15 = 30m Gold)
Tainted Gem x100 (1700)
Unidentified 13 (15)
You will get 1 Roentgenium each time you complete the quest. You need 15 to merge the class.