When you hold your action while casting a spell, if you hold till the start of your next turn, you can use your new action to continue holding that same spell, which now uses concentration. You can continue this by successfully making a concentration check at start of your turn. For every turn the spell is successfully held, the spells level increases by 1 up to the level of the highest spell slot you can generate after which each level increase takes twice the number of turns as the level before. The DC for the concentration check is 3 x the difference between the level the spell is about to be raised to & the level it was originally cast at.
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