Simply something like the block grid, but aimed into showing where is the void level, since there is no good way to do it exactly on the game.
By the way, the void level is also misaligned according to what is the actual building grid level.
For example, the grid on this picture is shown as being at the level 13 of map, but actually the void on the same level is shown as 26:
There is the character being instantly killed according to this void level as i’ve told about: