From LSWiki

This charm summons a moderately-powerful spirit of elemental fire. These are considered followers, and will provide additional damage in combat; also they provide protection by succoring the consort from enemies. They can be kept, and use the "Fire Creatures" keep capacity. They do not accept commands. Fire spirits fly and emit light.

With higher pyromancy skill, multiple fire spirits can be summoned. The level of the summoned fire spirits is 15 with minimum skills, and appears to scale up, possibly based on bond strength or pyromancy skill.

Your familiar must be alive and present in your environment for this charm to work.

This charm costs 100 points of elemental fire resource energy to activate.

Usage

concentrate on summoning a fire spirit