Heart of Deimos is a quest the centers around one of Mars’ moons, the titular Deimos. To begin the quest, you will need to download the Heart of Deimos update, then log into the game. The first thing you will see is a message from Loid, telling you that he needs help, and is holding you to old Tenno promises to protect Deimos.
One that is complete, you will need to begin the quest in the Codex. Head to the Codex module in the front of you ship and interact with it. Pick the Heart of Deimos from the list of quests to begin.
Now, go to the Navigation module, and click on Deimos in the Star Chart, which can be found directly beside Mars. Click on the Cambion Drift node, and you will be whisked away to begin the quest. You will need to complete this quest to get access to all the content in the Heart of Deimos expansion.
Load into the mission and make your way through the tunnels, and you will be contacted by a new NPC. Go the waypoint and destroy the Hive, then repeated the process at the next waypoint. When both of them are complete, your new allies will be satisfied that you are powerful enough to help them, and will grant you access to the Necralisk. Go to the new waypoint and you will have access to the game’s new social area.