I want so then entity dies, Player gets a wither effect, but I don't know how to do it.

it depends on if you want the entity to give the player wither when the player kills it or just whenever an entity dies if you want the first you can do this and use a hidden advancement of course you can probably just remove the condition for if you want it to affect every mob and make sure you make it give the effect to the source entity which is whatever kills the mob ie the player if you want it to specify the mob that was killed make sure to make it check the "event/target entity" and check if it belongs to that entity and if you want multiple entities that do this you can make a tag that includes all eligible entities hope this helps