A Celestial Sigil
Celestial Sigils can be obtained by winning in The Hall of Heroes, the final map in the Heroes' Ascent tournament. The Celestial Sigil is needed to purchase a Guild Hall.
If the leader of the guild (or someone in the guild who becomes leader just to buy the hall) won the sigil himself/herself in The Hall of Heroes, the flags in the hall will have the emblem of the guild. They won't have it if the sigil was bought or traded.
Sigils can also be purchased from the Sigil Traders located in Cantha and the Heroes' Ascent, which costs many platinum, based on the supply and demand. Finally, sigils can be traded between players.
For the Nightfall PvP event you can trade in 10 Flames of Balthazar for one.
Note: When buying a Celestial Sigil directly from other players, be aware of the Sigil scam!