In Elden Ring, Great Runes allow you to have major temporary buffs if you equip them, purify them at specific locations, and use a Rune Arc, expendable rare items.
What are Great Runes in Elden Ring?
@shiraziinspace, an Elden Ring player on Twitter and streamer on Twitch, explained how they defeated Rennala, the lord boss of the legacy dungeon Academy of Raya Lucaria, but ended up dying at the same time.
Moreover, as seen in the video clip below, seeing Elden Ring has an autosave system, the game now considers Rennala as defeated, so the boss fight cannot be triggered anymore.