Applying (adding) an effect on hit.

For example, there are Gas Elementals – I want to implement a visual effect of a cloud of death (the one that shoots larvae) when they attack (and when they are attacked and receive damage). There is an area-of-effect attack (I’ve implemented it), but without this visual effect, it doesn’t look very good. Also, there is a Hydra ability – specifically, when the creature attacks, the surrounding units take damage. It doesn’t seem too complicated; do you have any ideas or examples?