To cast this spell you must have in your service a familiar conjured from the find familiar spell. Upon  touching your familiar when you cast this spell, the familiar’s chosen form changes, growing into a large or huge creature at the casters discretion and taking on a fiery appearance. While in this form the familiar is considered an elemental, and gains the following benefits: 30 additional hit points, +2 to all of its stats, and the Fire Form ability.

Fire Form. The Fiery familiar can move through a space as narrow as 1 inch wide without squeezing. A creature that touches the Flaming familiar or hits it with a melee attack while within 5 feet of it takes 5 (1d10) fire damage. In addition, the Fiery familiar can enter a hostile creature's space and stop there. The first time it enters a creature's space on a turn, that creature takes 5 (1d10) fire damage and catches fire; until it takes an action to douse the fire, the creature takes 5 (1d10) fire damage at the start of each of its turns.

Your Fiery familiar acts independently of you, but it always obeys your commands. In combat, it rolls its own initiative and acts on its own turn. A familiar in this form regains the ability to attack, Dealing 1D8 piercing or slashing damage as appropriate as well as an additional 2D8 fire damage.

When the Fiery Familiar drops to 0 hit points, it disappears, leaving behind no physical form, however as long as your concentration hasn’t been broken it can be resummoned using a bonus action by sacrificing a spell slot of 2nd level or higher.

While your Fiery familiar is within 100 feet of you, you can communicate with it telepathically. Furthermore when you cast a spell you can choose to cast it as if your familiar had cast the spell. Your Fiery familiar must be within 100 feet of you, and it must use its reaction to deliver the spell when you cast it. If the spell requires an attack roll, you use your attack modifier for the roll.