Chaos Warp

You harness the magic used for a different creature's spell, fizzling it but also transmuting it into a random spell. What could go wrong?!

Roll a d10. Each number counts as a different level of spell, up to a level the original casting creature can cast. Cantrips are not counted. For example, if you roll a 2, the d10 lands on a 2nd level spell. If you roll a number higher than the creature can cast, for example a 9 and the creature can cast up to 5th level spells, you reroll the d10 until landing on a number of a level the creature can cast, a 5 or lower.

After the d10 has landed on a spell level the creature can cast, the DM counts how many spells of that level the creature knows or has prepared, and assigns a die and a number for each or most of them. For example, if a creature knows 11 spells of that level, the DM assigns a d12, each result assigned to a different spell, and if it lands on a 12, it is rerolled until it doesn't. You roll the die determined by the DM.

Once the spell level and specific spell have been rolled this way, the creature whose spell you fizzled, can cast or begin casting the spell (no action required) without expending a spell slot.