It is first seen on the Boss Monster's robe during the introduction scene to Undertale, and secondly seen on Toriel's robe, foreshadowing her role in the story, and the familiar symbol appears in other places.
There is a prophecy. The Angel... The One Who Has Seen The Surface... They will return. And the underground will go empty.
Ancient Glyphs in Waterfall
The Delta Rune is said to symbolize a prophecy that predates written history. However, the original meaning has been lost in time. What is certain is the bottom triangles symbolize the monsters of the Underground. The winged circle is believed to represent the "angel," which is prophesized to be the one that has seen the Surface and will return one day to depopulate the underground.
This prophecy has two interpretations: one where the "angel" destroys the barrier and brings freedom to the monsters, and one where it is an "Angel of Death" that kills everyone in the Underground.
Dialogue with Gerson
- Oh yeah... The prophecy.
- Legend has it, an 'angel' who has seen the surface will descend from above and bring us freedom.
- Lately, the people have been taking a bleaker outlook...
- Callin' that winged circle the 'Angel of Death'.
- A harbinger of destruction, waitin' to 'free' us from this mortal realm...
- In my opinion, when I see that little circle...
- I jus' think it looks neat! Wahaha!
- "Delta Rune" is an anagram of "Undertale."
- The Delta Rune bears a resemblance to the royal crest of The Legend of Zelda series, both with three triangles and a winged figure.
- The prophecy states that when an angel appears, the underground will be empty. This hints at the two main routes to beat the game, in which Asriel Dreemurr may destroy the barrier and free all monsters, or Chara may reawaken through the protagonist and kill all monsters.
- "Delta Rune" may mean several things.
- "That's the Delta Rune, the emblem of our kingdom. The Kingdom... Of Monsters." - Gerson