Problem Creating a relic

How my campaign is going I need to create a relic that they are going to obtain in this case the trident of posiedon. I am trying to figure out how to make this so that it would be better than just a trident of fish command as a character in my group already has that. But i do not want to make it game breaking. I want it to be very powerful but have some sort of downside since it wasn't made to be used by mortals. 


Thanks in advance, 

Sincerely,


Nate Brown