Upon activation, Isaac is turned into a demon and gains the effects of Brimstone for the current room.
If Sulfur has already been used in the current room, it instead gives Isaac a 1.2x damage multiplier, +1 damage, and increases the size of the laser for the current room.
Further uses of Sulfur will increase Isaac's damage by 1.
This item belongs to the Leviathan set. Collecting three items from this set will transform Isaac into Leviathan.
Notes
Sulfur functions identically to Brimstone. For a full list of interactions, see that page.
Sulfur is extremely powerful in Greed Mode, since it can be used multiple times in the same arena.
Synergies
Book of Virtues: Spawns red wisps in the middle ring that fire a short Brimstone laser beam similar to Azazel, but significantly smaller.
Brimstone/Car Battery: Activates twice to give an additional damage bonus and wider laser.
Sulfur is from the Antibirth mod. Originally in Antibirth, due to a bug, using Sulfur twice in one room would permanently grant Brimstone to Isaac instead of giving him a more powerful brimstone laser.
This item originated in the "Binding of Isaac: Community Remix" mod for the original Binding of Isaac game.
"Sulfur" and "Brimstone" were once synonyms but the word "brimstone" has fallen out of use (outside of biblical usage).
Sulfur's appearance is based on the alchemical symbol for sulfur, although the triangle is upside-down and it lacks a small piece separating the cross from the triangle.
Brimstone for the current room.
Leviathan set. Collecting three items from this set will transform Isaac into Leviathan.
Book of Virtues: Spawns red wisps in the middle ring that fire a short 
Azazel, but significantly smaller.
Car Battery: Activates twice to give an additional damage bonus and wider laser.
The Nail.
The Binding of Isaac: Rebirth
