You use the following Metamagic options to temporarily modify spells you cast. To use an option, you must expend the number of Hit Point Dice that it costs. You can use only one Metamagic option on a spell when you cast it unless otherwise noted in one of those options.
Empowered Spell (Costs 1 Hit Point Die)
When you roll damage for a spell, you can spend 1 Hit Point Die to reroll a number of the damage dice up to your Charisma modifier (minimum of one), and you must use the new rolls.
You can use Empowered Spell even if you've already used a different Metamagic option during the casting of the spell.
Quickened Spell (Costs 2 Hit Point Dice)
When you cast a spell that has a casting time of an action, you can expend 2 Hit Point Dice to change the casting time to a Bonus Action for this casting. You can't modify a spell in this way if you've already cast a level 1+ spell on the current turn, nor can you cast a level 1+ spell on this turn after modifying a spell in this way.