When I saw this sentence first, I thought so powerful to be an objective complement. [ ‘so powerful’ is the result of ‘make’] But now, it may be more reasonable to think ‘so powerful’ is modifying ‘a sleeping potion.’ Can the adjective phrase ‘so powerful’ be a postmodifier?
For your information Potter, asphodel and wormwood make a sleeping potion so powerful it is known as the Draught of the Living Death.