Note that the only truly devastating move that Darius possesses is his Ult.
Darius in particular is a single target champ that is best if he can keep the enemy target close by. His most underrated move is his E which is a pull with a surprising range. If Darius manages to pull 2 or more of your team in and land a perfect Q his team
is gona eat yo brains for breakfast even before you can say boop. The trick against Darius is to get him stunned. There are actually allot of builds out there that focus on tanky Darius with stun and snare reduction because once again if Darius gets close your pretty much over.
t.l.d.r. : The key counter against Darius is to stun or silence him. Stun him before he can pull you in or silence him right after he has pulled you in.
This gives you time to get the distance you need to get. An ideal champ would be anyone with an escape or a lunge. For example Xin Zao with his ult, tristana with her cannonballs, etc. Another option would be to silence him and then mass focus him with your team. A great option would be to ult him with WW and then unleash hell.
HOWEVER, getting back to your question you stated a fed Darius in a teamfight. Im assuming a 5v5. Well it depends what the team composition is, if your got a tank on your team with a ranged attack, like Mundo try soaking his teammates. If you have a blitz or anyone with a decent pull, pull the squishy units in but only when they are out of position.
Ultimatie t.l.d.r. :
When encountering a overfed Darius in teamfights. Always target the squishy units first just like in any other teamfight (never target the tank). If Darius over extends and you have a pull, pull him in and let your team finish him quickly. If you cant kill the darius with your entire team without him having support from his team you need to reconsider your chances of actually winning. Most importantly stay passive. Dont engage, let them engage and make mistakes. Also dont commit, you dont need a more fed Darius then you already had.
I think this pretty much covers it, it's not easy to give a strategy unless you actually have all the champs to talk about but in short a tanky Darius is never the one you want to engage first in teamfights. Stay out of reach of Darius his pull. And have a lot of fun!.
Hope this helped,