Equilor was one of the most ancient and distant planes in the multiverse. According to Urza Planeswalker, it was the most ancient plane, from the "edge of time".  
The plane was inhabitated by powerful beings, who were seemingly human. It is not known if they were planeswalkers, or just a civilization that developed powerful magic. They claimed that Urza found them only because they allowed him to do so. Supposedly, their magic could conceal the plane. They also had much insight on other planes in the multiverse.
A notable location is Bloodhill Bastion, a city on a hill that seemingly is created from living tissue. Sphinx-like creatures are guarding it.
During the period of the Shard, Urza traveled there in hopes of discovering the origin of the Phyrexians. The elders of that plane told him that the Phyrexians couldn't reach Dominaria because of the Shard of the Twelve Worlds. They also cautioned him that Dominia is full of dark forces like Phyrexia, but that they will fall eventually. Calmed by the revelations of the Equilorians and the splendor of the place, Urza decided to stay on Equilor. He was amused by the knowledge the people of the world had gathered along its eons of existence, and was also astonished that the Equilorians left it almost unused. Urza quickly devised many ways to employ it, and wished to change Equilor into a mechanical paradise, but his intrusive plans weren't received well by the majority of the elders. Fortunately, Urza soon had to leave their home plane. The Shard had been destroyed, which again left Dominaria unprotected from Phyrexian forces. Once the elders told him of this, Urza planeswalked away to prepare Dominaria for certain invasion, never to return to Equilor.