Destiny 2 Theory Provides Best Guess at What 'The Veil' Is

gamerant.com:

Destiny 2 players frustrated by the lack of detail regarding the Lightfall campaign's strange MacGuffin, «The Veil,» have formed a theory regarding what it truly is. The Veil plays a hugely significant role in the campaign of Lightfall as the goal of the expansion's key villain, The Witness. Despite that, and to the significant frustration of Destiny 2 players, The Veil is never explained throughout the campaign. It's left as a complete mystery, despite its significance. Destiny 2 players are trying to fill the gaps, but nothing is certain.

The Destiny 2 Lightfall campaign is preceded by a cinematic trailer revealing an encounter the live-service game has been building toward for some time. The Black Fleet has arrived at Earth with its leader, The Witness, at its front. The Traveler blasts The Witness with a ray of light, but the Witness is unharmed. The Witness goes on to seemingly commune with The Traveler, which points him toward The Veil on Neptune. That's as much as Destiny 2 players are told before Lightfall's campaign. The Witness wants The Veil, and the Guardians must stop him.

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Without delving into spoilers for theLightfallstory campaign, a theory has been formed by Destiny 2 players regarding what The Veil is. In a Reddit post from user tunnelinggeek, with reference to posts from K1dP5ycho and Rorshark, The Veil is described to be a «literal link, a transporter, between the paracausal forces of Light and Dark.» In other words, it's a physical representation of a metaphysical veil between the Light and the Dark, which The Witness seeks to pierce.

That is the conclusion of the theory, but there's a good amount of evidence worth considering in order to reach that