To get the Esper "Crusader" you must destroy all 8 Dragons, Phunbaba, and DoomGaze.
If you do these tasks, the Dragon Seal will shatter.
Blue Dragon - In same room as stone queen
Dirt Dragon - Opera House, on stage (hit control room switch)
Gold Dragon - Kefka's Tower
Ice Dragon - Narshe, in snow fields
Pearl Dragon - Fanatics Tower, in treasure room
Red Dragon - Pheonix Cave
Skull Dragon - Kefka's Tower
Storm Dragon - Mt Zozo
DoomGaze - Randomly attacks airship, while flying in air
Phunbaba - Attacks immediately after Terra joins party
You can only complete these tasks in the World of Ruin.