Knight Astlin's holotape is a holodisk in Fallout 4.
Location
It can be found on the body of Knight Astlin within the National Guard training yard and is relevant to the quest The Lost Patrol.
Transcript
Knight Astlin: Knight Tara Astlin. Brotherhood of Steel Recon Team 429-Alpha. Serial number 3431. It's been three hours since I set my distress pulser. There's been no word from the Paladin or Faris. Their objective was a satellite array on the coast. They may be out of range. My orders were to hold this position at all costs. The entire site has been overrun. The door won't last much longer. Paladin Brandis, sir. It's been an honor, sir.