It looks like the overlay was most likely an Elden Ring boss/death tracker overlay , the kind that shows boss status, kills, and death counts on stream. A matching example is the “Elden Ring Boss Tracker,” which specifically tracks whether bosses are alive or dead and can show counts for bosses killed and remaining.

What it probably was

  • A stream overlay layered in OBS or similar streaming software, not something inside Elden Ring itself.
  • The overlay’s purpose was to keep a running tally of bosses, deaths, and progress during the marathon.
  • Some versions are direct game overlays, while others are website-based trackers or streaming widgets.

Closest known match

The closest public match I found is Elden Ring Boss Tracker from Nexus Mods, which offers:

  • boss alive/dead tracking,
  • area-based boss lists,
  • kill and remaining counts,
  • a toggle hotkey for hiding the overlay.

Caveat

I couldn’t verify a public post from Jinxzi naming the exact overlay he used, so this is the most likely match rather than a confirmed one.

TL;DR

Most likely: an Elden Ring Boss Tracker / death-counter style overlay used in streaming software to track deaths and defeated bosses.