Mimiron's Sigil and its Heroic mode equivalent are a portion of the third step in the Algalon the Observer attunement quest chain. Completing this quest requires the defeat of Mimiron in his Hard mode: Activate the timed self-destruct sequence before starting the encounter.
For more information on obtaining Mimiron's Sigil consult the Archivum Console.
Mimiron's a clever one. His entire abode is rigged with some sort of self-destruction mechanism.
Unfortunately yer gonna have to race against it if ye want to get his sigil. Check the Archivum Console for more details if ye need them.