Seasonal Pennants are granted to players who participated in Seasons in Diablo III, beating Chapter 4 of Seasonal Journey. Not every Season offers a Pennant reward: so far only three, Seasons 3, 6 and 10, have such rewards, while other Seasons had pets instead.
When obtained, Seasonal Pennants can be added to Wardrobe. Only one pennant can be used at a time. If equipped, it will be visible even in character selection screen.
Season 3 Pennant does not have a special name. It is fashioned in form of Diablo's head on a pole, with the game's three vertical stripes on a black banner.
It also comes with a portrait frame of similar design.
Third time is the charm.
You braved the depths of Season 3, and this is what you found.
Season 6 Pennant is named Samhain. It is fashioned in form of a Reaper scythe, with a Reaper's hood on top, spine as shaft and a silver demon's head on a pale blue banner.
On the last day of harvest season, the wisest folk lock their doors, hang their wards, hide under their beds, and wait for dawn.