Delta Rune

The Delta Rune is an emblem representing the Dreemurr royal family. It shows an orb between two wings and three triangles beneath it, with the middle triangle inverted.

It is first seen on the Boss Monster's robe in the introduction to Undertale, and secondly seen on Toriel's robe, foreshadowing her role in the story, and the familiar symbol appears in other places.

Symbolism
The Delta Rune is said to symbolize a prophecy that predates written history, although the original meaning has been lost in time. The only thing that can be said with certainty is that the triangles below symbolize the monsters of the Underground; although many believe the winged circle symbolizes an "angel." The prophecy narrates of an "angel" who has seen the surface, who will return to the Underground and bring the monsters freedom.

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 thing 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!

Trivia

 * "Delta Rune" is an anagram of "Undertale."
 * The Delta Rune bears a resemblance to the royal crest of the 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.
 * The True Pacifist route makes the angel free the monsters, and they leave the underground to the surface, leaving the underground empty.
 * The Genocide route makes the angel of death kill every monster, leaving the underground empty of its inhabitants.