We first see Jack like that in one of the episodes, I thought it was a first season episode but I'm mistaken it's from Ep. 32 "Jack and the Travelling Creatures". The creatures help Jack get to a portal where Jack is confronted by the portal's Guardian and they fight; one of the final shots is of the Future Jack above.https://youtu.be/VS1mo28G-Ow?t=42s
The new season has gotten me interested in maybe tracking down the comic (not as simple to do without a comic shop in town). I haven't looked yet, hopefully the Samurai Jack comics have been collected into a couple TPBs.