Spellstealer

Your innate magical abilities and keen mind allow you to easily weave spells that other creatures cast by simply observing them. 

On any turn other than your own, whenever you see a creature cast a spell, you can use your reaction to observe them and potentially copy it. The spell must use no more than 50gp worth of materials, be at a base level of 5th level or lower, and you must be able to see the verbal and somatic components of the spell when it is cast. 

Succeeding a DC (10 + The spell's level) Intelligence check allows you to cast the copied spell at the same level it was originally cast as an Action or Reaction within the next minute. As if you are concentrating on the spell, you can not cast any other spell unless you stop concentrating.

After casting the spell or losing the ability to cast it, this feature can not be used again until you finish a long rest. Failing the Intelligence check consumes your reaction but does not consume the use of this feat.