The liquid void is a block added by the Xycraft mod. It has multiple uses, the main being for Xycraft multitanks. If placed in a multitank, liquids will not be able to rise above it in the tank. For example, if you make a 5x5 multitank, and place the liquid void at the 3rd level, you will only be allowed to fill 1 row with liquid (because the bottom level holds none).
Additionally, liquid void can be placed in the world to prevent liquids from touching it. This can be useful for mining, where the liquid void can be placed around lava to destroy it. It can also be used to make lamps, where a block is placed on top of the liquid void and lava is placed on top of that block. The source will not be destroyed, but lava will flow off each side, but won't be able to pass the liquid void and will simply float in the air.