Ragnaros by MarioToast

Level 1

Wildfire: Ablaze spreads 50% further.
Headbonk: The hammer in Sulfuras Smash stuns enemies for 1 second.
Consuming Fire: Quest: Hit Ablaze enemies with Magma Blast to increase its damage by 2, up to 50.
Reward: After hitting 25 Ablaze enemies, also increase the explosion radius by 33%.
Empowered Regeneration: Regeneration globes restore 75% more Health and Mana.

Level 4

Melt Armor: Ablaze Heroes are Vulnerable, taking 25% increased damage from all other sources and revealing them.
Fuel For The Fire: Basic Attacks against Ablaze enemies deal 35% bonus damage and increase the duration by 1 second.
Crippling Heat: The crack left behind by Sulfuras Smash slows enemies by 40%.

Level 7

Burning Burst: Immolate sets enemies Ablaze in a short range around each target.
Hammer Throw: Activate to make your next Basic Attack ranged and deal 97.18 (+4% per level) bonus damage. 10 second cooldown.
Follow Through: After using an Ability, your next Basic Attack within 6 seconds deals 40% additional damage.
Homing Fire: Magma Blast becomes targeted instead of a skillshot.

Level 10

Volcanic Eruption: Summon a giant eruption of lava.
Sons of Flame: Summon 8 Sons of Flame to fight alongside Ragnaros.

Level 13

Intense Heat: Increase the damage of Engulfing Flames by 20%.
Fire Trail: While mounted, you leave behind a trail of fire that lasts for 4 seconds, dealing 53.24 (+4% per level) damage per second, slowing by 25% and setting enemies Ablaze for 2.5 seconds.
Arson: Engulfing Flames deals 200% bonus damage to Structures.
Literally Burning Rage: Deal 19.67 (+4% per level) damage per second to nearby enemies. Sets enemies Ablaze for 1 second after 4 seconds.

Level 16

Incendiary Bomb: If an enemy Hero dies while Ablaze they explode after 0.2 seconds, dealing 346.17 (+4% per level) damage to nearby enemies and setting them Ablaze for 3 seconds.
Rapid Fire: Reduce Magma Blast's cooldown by 3 second and give it an additional. Damage is reduced by 40%.
Kindling: Minions and Mercenaries can spread Ablaze as well.

Level 20

Blackrock Mountain: Volcanic Eruption deals 50% increased damage. Cast time is reduced to 0.2 seconds.
Brotherhood of Flame: Summons 2 additional Sons of Flame. Sons of Flame have 100% bonus Health and set enemies Ablaze for 1 second.
Unbearable Heat: Ablaze enemies are slowed by 30%.
Bolt of The Storm: Activate to teleport to a nearby location.