Caustic Blood

Even the act of slaying a Tyranid can prove fatal, as
its corpse spews corrosive ichor capable of eating
through power armour.
Use this Stratagem at the start of a Fight phase.
Select a TYRANIDS unit from your army. Rol
l a dice whenever a model in that unit is destroyed
in this phase. For each roll of 6, the enemy unit
that inflicted the final wound on that model
suffers a mortal wound after all of their attacks
have been resolved.