The quest "The First Firewall" is available once all four fusion rifle relics are found and returned to the gunsmith, it is available on Earth as seen here: http://i.imgur.com/aLogKZu.png
Completing the quest awards the Curious Tranceiver, which goes to your quest items and then requires 4 sequential passcodes. The passcodes appear to be identical to the order the knights spawn in the First Firewall mission (with Knights and Taken knights representing different values).
Tranceiver passcode input screen: http://imgur.com/NGvBxbB
All codes courtesy of /u/kiki_strumm3er:
Codes (top is 1):
- 1 2 1 1 2 2 2 1 2
- 1 2 1 1 2 2 1 1
- 1 2 2 2 1 2 2 2 1 1
- 1 2 1 2 1 2 1 1
After 4 passcodes you get access to a new mission called Shadow Call which requires you to reach the top of the Devil's Spire (Cayde's Stash mission location) within a time limit to collect your reward, which is the IKELOS Fusion Core.
The Fusion Core requires 5 modules to be repaired.
Heavy Power Coupling - Dismantle any legendary or higher heavy weapon.
Ionized Shell Harmonics - Complete the Archives mission on Venus on any difficulty.
WARSAT//MK19/TERRA/RSPN - Complete a 'Defend the Warsat' public event on Earth.
WARSAT//MK19/LUNA/RSPN - Complete a 'Defend the Warsat' public event on the Moon.
WARSAT//MK19/MARS/CHLM - Complete a 'Defend the Warsat' public event on Mars.
For help finding Warsat events, check out www.destinypublicevents.com. Note that the Warsat at the beginning of the Fallen S.A.B.E.R. strike does not count towards your Earth Warsat completion.
Completing all 5 modules awards you with the Reassembled IKELOS Fusion Core, which must be brought to the Gunsmith. Turning it in results in a dead end, and it seems most likely that the next step is time gated.