Liquid death is a fixed-volume fluid. Once placed, it will flow into adjacent squares, but will not create more fluid like a lava or water source block. The resulting partial fluid blocks cannot be picked up with a bucket, but can still be destroyed by replacing them with another block. Only if the fluid block is still filling a single block space can a bucket collect it.
The damage done to a mob is proportional to the (partial block) depth of fluid they are standing in. The essentia crystals dropped are in addition to the mob's usual drops for environmental damage. Collecting the drops may be non-trivial, but hoppers can be helpful.
Put a bucket into a crucible. The crucible should contain 32x Perditio, 32x Mortuus and 32x Venenum.
Thaumcraft 4.2 Tutorial [Liquid Death]
This video was made by Zautos