The quick, easy, utterly reliable magical oath - frequently of the form "So-and-so pulled out his wand and said, 'I swear on my magic that this is the truth. So mote it be.'" - with the penalty for swearing falsely being loss of the oathgiver's magic.
The notion that when the prophecy says "he shall have power the Dark Lord knows not", it's referring to a singular, specific ability - "a power", rather than "power".
The notion that when the prophecy says "he shall have power the Dark Lord knows not", it's referring to a singular, specific ability - "a power", rather than "power".