Does anyone know how could I make it so that an entity gives a potion effect to all the entities it attacks?

I think you can make a procedure with a global trigger of "when entity attacked". And then check if the source entity is your custom entity, and if it match, give the potion effect to the target entity (the attacked entity)

In this example procedure:

- We get it triggered as soon as an entity gets damaged in the world
- We check if the entity that griefed the damaged entity is a Bee
- If its a bee, then, we give a Fatigue Potion Effect to the damaged entity

Hope it helps ^^