You must be able to cast spells of 3rd level or higher to take this feat.
Through much practice and true skill, you can bend magic to your will, casting multiple spells in quick succession much faster than your average caster, but not without a cost.
After casting a spell with your action, you may then cast another spell as a bonus action. However, there is a trade-off, as sacrifices must be made for power.
If you use your bonus action to cast a spell after you have used your action to cast a spell, you must immediately expend 1 hit die (starting with your smallest die if you are multiclassing). You then take damage equal to 1 + the level of the spell you cast with your bonus action. Casting a cantrip as a bonus action causes you to take 1 damage. This damage cannot be reduced in any way.
If you do not have any hit die remaining, then you cannot use either the feature granted by this feat.
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