You have altered the Spell Steal spell to hold yours or someone elses spell and charge it up. When you see someone cast a spell within range, or when you cast a spell, you can catch it and/or contain it, as you pour more and more magic into it. when you do you make a Spellcasting save DC 10 + the spells level you are trying to upcast it to, but can only increase it by one level each round. if you succeed, you can send it out as a bonus action, as it is already casted and just needs to be sent, or you can hold on to it more, and charge it with your action on your next turns. if you fail, you take the original spells damage before it powered up

For instance, If you are to cast or someone casts fireball at 3rd level on you, you can catch it and charge it. you spend your turn and your next turn to charge it, and send it out after the 2nd charge, you now send out a 5th level fireball. but if you are to fail, no matter when or how many charges you gave it beforehand, you get hit by it, whether it has a save or not.