Demon Firesage

The Demon Firesage is an optional boss in Dark Souls. Its appearance is similar to that of the Asylum Demon and Stray Demon.

Location
The Demon Firesage is found in the Demon Ruins, down the staircase from the second bonfire and past the Taurus Demon. In order to enter the fog gate, the Lordvessel must have been placed.

Lore
It was thought that the Demon Firesage was "the first demon to ever exist, and before becoming a demon, was the last master of the original fire arts", however, the existence of Ceaseless Discharge gave doubt upon this statement. Later on, it was revealed by Miyazaki that the Demon's Catalyst was improperly translated; for it mentions that the Firesage was the master of Chaos Pyromancy and not the original fire arts - indeed, for it was Ceaseless that was the first Demon.

Strategy
The Demon Firesage is very similar in terms of moveset and attacks to the Stray Demon, so the same rules apply here: staying behind it and keeping a shield up should damage be unavoidable are good ideas. Be wary of its overhead attacks that are blockable, but deal heavy stamina damage.

The Demon Firesage is highly susceptible to Bleed buildup.

Getting too far away from it will cause it to either leap at the player with its hammer, or do a straight down smash, which can take away a large chunk of stamina. Note that both of those attacks do not track, so with proper timing, one can easily roll through their smashes.

Butt Slam
It will fly up into the air to perform this attack.

Exploding Swing
It will only affect the area in front of it and being behind it is a safe zone.

Exploding Hammer Drive
It smashes its hammer into the ground. Affects the area all around it.