Arcane Mimicry

You observe the spell cast by the creature and imprint the spell in your mind, when you cast this spell the DM must tell you the level of the spell you are trying to mimic, it consumes a spell slot equal to that level, if you do not have a spell slot of a high enough level the spell fails and consumes the highest level spell slot available to cast. 

While you maintain concentration on the spell you can cast it using the appropriate action on your turn (or as a reaction off your turn) once you cast the spell in this way you drop concentration and the memory of the spell is lost. 

As long as you maintain concentration on the spell you can copy the spell into your spell book following the same rules as copying a spell from a spell scroll.