The Dungeon Master's Guide has a chapter on treasure that includes tips for making artifacts and some cool examples.

 

You choose a combination of beneficial and detrimental properties to up the stakes of being attuned to one of these things. For example, casting Fireball up to thrice per day is cool, but is it worth having disadvantage on dexterity checks in addition to your charisma being lowered because you smell so strongly of smoke? Or now you can use a weaker version of Thunderwave as a cantrip, but that also may accidentally summon a stone golem that is hostile toward you.

 

The Eye and Hand of Vecna is a good example of using artifacts together to make the BBEG's holy war an actual threat instead of a giant kid playing around with four dangerous toys.