Rhakotis

 was an alien who crash-landed in Egypt in the 4th century BC during the reign of Alexander the Great.

At first the Egyptians were hostile and wanted to kill him, but one person, an amnesiac Barbara Wright, was able to translate for him. He asked for help for his injuries and promised technology in return.

One of the things Rhakotis gave the Egyptians was time portals in the library, through which people could travel back and forth to various times. Eight years after the crash, Barbara went through one of these portals and arrived just as Rhakotis had crashed.

When a power struggle ensued, Ptolemy Lagus, whose position as king was disputed, asked Rhakotis for help. Rhakotis' attempts to help destroyed the city, killing many, including Ptolemy and Ian Chesterton. The First Doctor showed Rhakotis and Barbara how they could erase this timeline, but it meant that Rhakotis had to sacrifice his life. (PROSE: The Book of Shadows)