
You can summon divine fire to your weapon.
- Does an additional 2d6 + CHA modifier damage plus normal sword damage.
- Will ignite flammable material upon contact.
- Lasts up to 10 minutes or stopped by character.
- Sword's flame cannot be extinguished by non-magical means.
- Can only be used once a day until a long rest

How is it activated and you should really specify these things in the feat
Thank you for the question.
It is activated by a verbal command as a bonus action.
"Does an additional 2d6 + CHA modifier damage plus normal sword damage". This implies you are adding 2d6 + charisma modifier AND the sword's base damage to the sword. Correct me if I'm wrong, but I don't think you wanted it worded that way. (Sorry if I seem a bit snarky here, that's not my purpose with this comment.)
When this was created it was the 2d6+ CHA mod plus sword damage. So a long sword will do d8 +damage modifiers PLUS the 2d6 plus Char modifier. I have had parties change it to be less powerful, but that was the original intent. And no, not taken snarky at all.
May i Introduce you to the magic item, The Sun blade: https://roll20.net/compendium/dnd5e/Sun Blade#content It is worse, but if you take this feat at level one as a variant human, you can get something MORE POWERFUL THAN A RARE ITEM, which is very broken. it should just be the 2d6+cha to make it Relatively balanced.