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 Seasons 3, 6, 10 and 14 have such rewards, while other Seasons had pets or wings 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.
The 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 / Season 18
The Samhain pennant was available as a reward in Season 6 and 18. 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.
Season 10 / Season 22
The Loremaster pennant was available as a reward in Season 10 and 22. It is inspired by the Book of Cain, with the shaft in form of a cathedral chandelier, and the banner itself being a large Horadrim scroll with a sigil on it.
You are a worthy successor to Deckard Cain.
Justice shall be done.
For the nephalem hero who wants to embody the spirit of Justice, a brand-new pennant dedicated to everyone's favorite archangel will also be available. Just remember — you cannot judge Justice itself!