blue-Flame Blade

choose a melee weapon you are holding the weapon or the weapon's blade (your choice) turn into magical blue flame for the duration.

for the duration you gain the following benefits:

• the weapon's damage type is changed to fire damage.
• the weapon deals an additional 1d4 fire damage. 

additionally you can use an action to create a 15 ft long 5ft wide line each creature in the line must make a dexterity saving throw or take 1d8 fire damage, the flames ignite only item that are not worn nor held. after you use the spell in this way the spell ends.

if you stop holding the weapon the spell ends.

This spell’s damage increases when you reach higher levels. At 5th level, the weapon damage is increased by 1d4 fire damage and the line damage is increased by 1d8 fire damage. 

both damage roll increase by 1 die of the same type at 11th level and 17th level.