I still don't know what you want. I can't tell you which event. I don't know when this should happen oO Just try to say what you want to achieve, maybe this helps :/
EDIT: Ok, due to your post I guess:
a) Do you want it to happen whenever someone comes near to your unit
b) Do you need enemies and allies when a spell is cast
c) Do you want to pick all enemies and allies periodicly?