Star Trek: 10 Things We Now Know About The Temporal Wars

The Temporal Wars changed the being known as the Guardian of Forever. When it was first encountered by Starfleet, it only held that circular form, yet after the wars, it had evolved to manifest as Carl, a humanoid means of interacting with potential users.

Carl explained that during the Temporal Wars, it had been repeatedly abused by factions attempting to change the course of history. In The City On The Edge Of Forever, the Guardian described itself as a tool, something that travelers could use to pass into the pages of history. It retained this purpose by its second appearance in Yesteryear.

The abuse it received from Time Soldiers and Temporal Agents seems to have mellowed it, with Carl choosing to fully evaluate Emperor Georgiou before allowing her to pass through. Carl also seemed to have the power to send her across dimensions, a skill that was previously unknown.

Who, or what, originally built the Guardian remains unknown. It is many eons older than even Earth's sun and shows no signs of aging out. It may still be the best means of travel between timelines, though only if one manages to convince Carl that they're worthy.