Stupid question time - are Mjolnir and Vibranium Shield action dice? For some reason I had them in mind as 'Hero' dice.
Yes, they are actions.
From the rules:
The special sides on action dice have a custom icon, and some have one or two burst symbols on them. Action dice never have numbers.
Character dice have a custom icon and may have burst symbols, but they always have several numbers that define the character’s basic abilities.
They don't have stats and you can't field them.