When your whole team die in an Instance a Message Box will pop and ask you, if you want to revive them and continue the instance.

You don't use them during a Fight.

When you're in an Instance and you retreat or your whole Team gets killed, you will see a Window pop-out that lets you use a Revival Potion. It's like a second chance to complete the Instance.

Thanks for the info, was curious what it was actually for.

is there any merit on using them? should i just sell them since dying on an instance means its not going to be a 3 star run anymore.

I only died once, and it was useful, but maybe you will need those in a future, better keep

Will it be usable for survival trial? for reviving the defeated ninjas.

I wonder the same thing.
They only sell for 1 copper coin as well. Hehe. :P
I mean, what exactly is the point in using them when you can just simply exit the instance and return again?
You don't waste any energy, and you are eligible for nabbing all x3 battle points. . .
If you do need to use the revival potion, you no longer are eligible for all three battle points . . .
Eh . . .