Each week, three missions will be invaded by the Black Tusk faction. The Tidal Basin stronghold will always be invaded because it is their main base of operations. Only invaded missions and strongholds can be played in the Heroic difficulty mode.
When an invaded mission is done, you might not find the Heroic difficulty mode by changing the difficulty manually.
To find the Heroic difficulty mode again, you will have to press the corresponding key or button in order to Toggle Invasion.
If you have any other questions, don't hesitate to contact us!