On Agents of S.H.I.E.L.D. Season 3 Episodes 21 and 22, Hive prepares to unleash his Inhuman gas upon the world and the fallen agent is finally revealed.
The "Fallen Agent" is finally revealed on Agents of S.H.I.E.L.D. Season 3 Episode 22.
The person to make the sacrifice for the cause is Lincoln.