Savitar is one of Barry's time remnants, meaning that if Barry runs back in time he can create temporal duplicates of himself. In order to defeat Zoom in last year's season finale, Barry created a time remnant. This time remnant ended up sacrificing himself to stop Zoom and save the multiverse.

How did the Savitar loop start?

In 2054, time remnant Barry reveals himself to the flash as Savitar, starting a war. This is the reason that Barry sent the warning to Rip Hunter. Barry defeats Savitar by trapping him in the speed force, thereby making Savitar immune to changes to the timeline.

How did Savitar get his scars?

You remember when future Barry told the real Barry that savitar killed all the the remnants he made, but it was a lie savitar didn't kill one of the time remnants but I think he beat him up gave him the scar then after iris died the time remnant came back to star labs but Barry disowned him that he was useless and that ...