How to make the player get the effects?

What effect

i think it’s just so dam easy u shall try urself

MatthewX, it's actually not very easy lol. I can say this because there's no trigger for "entity is killed with tool" and it's only possible through an event-based (globally triggered) procedure. plus, not all people are skilled at procedures. you didn't even teach him exactly how to do such thing, so... let me take it from here, i usually know what i do with procedures yet kind enough to show/teach people how to do something with them :D

Ball-van, you can use this as the basis for making your custom sword give players regeneration after they kill a mob with it:

all you need to change is the sword on the item procedure block to your own custom sword (as you can see i used netherite sword as an example). that's it, hope this helps! 

additional info for future reference:

• "event/target entity" in most cases refers to the mob that is the subject of an event (e.g. right-clicked or damaged)
• "source entity" on the other hand refers to the mob that is doing an action to another mob (like in this case, the player)
• there are times when target entity and source entity both appear in a procedure's provided dependencies. always remember that the event/target entity in this situation is the one receiving the action, and source entity is the one doing the action