Invulnerability is simply calling a function that makes the character invulnerable. For example, make a boolean called Invulnerable. It should start as false. When you roll you call a function that sets the variable to true. Then, when the roll is done you can set the variable to false again.
Then, where you handle damage taken, simply use a branch where you only take damage if Invulnerable is false.
This is not very adaptive or powerful way to implement this, but it is the easiest to understand and it will get you started. You might want to look into the node Delay for some control over the duration.