If you've spent at least $9.95 on ACs or membership, you can claim the FireLord Summoner Bonus Package from your online account manager. It rewards the FireLord Summoner class, which is a duplicate of Blaze Binder.
Otherwise, purchase the Darkness Sigil and Flame Sigil from High Adept Caustus's Pyromancer Sigils shop. This requires Pyromancer to be in your inventory with at least 1 Class Point, as well as Rank 10 Doomwood and Rank 10 Embersea. Then, merge the class from her Merge Shop.