Head paladin's keycard

The Head Paladin's keycard can be acquired by pickpocketing it from the Head Paladin, who may be either Hardin or Ramos, depending on your choices during Still in the Dark. The Head Paladin is in the Brotherhood of Steel bunker in Hidden Valley.

Uses
The Head Paladin's keycard is one of the 3 keycards required to start the self-destruct sequence during For the Republic, Part 2, Render Unto Caesar, The House Always Wins, and Wild Card: Side Bets.