The Fundamental Laws of Magic are, presumably, a number of statements about the general nature of magic. They were formulated by the magical theoretician Adalbert Waffling, the first of which is quoted above. It's unknown how many Laws Waffling created.
Possible interpretations of the first law
The exact consequences mentioned in the first law are not known, only vaguely stated to be "of the most extreme and dangerous kind". However, during Albus Dumbledore's discussion of the first law in his notes on the well known fairytale "The Warlock's Hairy Heart" (in The Tales of Beedle the Bard), he mentions that "in seeking to become super-human this foolhardy young man renders himself inhuman".
Likewise, Dumbledore also notes that the creation of a Horcrux evokes the first law, tampering with the essence of self or rather "dividing what was clearly not meant to be divided...body and soul". Voldemort created the number of Horcruxes he did solely in order to fulfil his perennial goal of evading death. Therefore, when he was finally killed the result is almost as karmic as the aforementioned warlock's: in seeking to fix himself immutably to one plane forever he renders himself eternally removed from all of them, save "the between-space".
Therefore, it may be that the consequences of the first Fundamental Law of Magic (when evoked) mimic Newton's Third Law of Motion: For every action there is an equal and opposite reaction.
In this light, it has been suggested that Merope Gaunt may also have been a victim of this law - she is suspected by Dumbledore of having been using Amortentia to tamper with affairs of the heart, which appear to be inextricably linked with the essence of self in the Warlock's Hairy Heart (and indeed, love is counted amongst the deepest mysteries of magic).
This theory is supported even further by the grave and ironic consequences that Merope is seen to have incurred in her quest to magically manufacture true love, e.g. driving away forever the very person she sought to be eternally bonded with, as well as rendering herself devoid of all positive feeling (depressed) in her attempt to reproduce one particular emotion above all others. Likewise, the only tangible thing her struggle for a union of true love ever produced was a being completely incapable of even comprehending the notion, who would go on to preach a campaign of hatred long after her death.
Evokers of the first law
- The Tales of Beedle the Bard (Mentioned only)