Lets think...
Luna: OK
Luin: Very Tiresome to do an if command and have ANOTHER trigger to cut the damage of each spell
Mana: Eh it's pretty simple
Thor: That's a bit too specific make it more simple
Kain: Same as Mana Moon
Merlin: Same as Luin but think about balancing problems...
Folt: just use an aura that appears somewhere on your hero
Pyrl: Uh 5 is a bit much
Yggdrasill: If we were to put the percent in decimals you would get .005 of a chance to get an "insta-kill" but it would be very unlikely to happen and would it be on every hit? Therefore if you have a superfast attack speed and an enemy with super high health then it would just be about luck then as HeZ said.
Overall it's a pretty original system but there SHOULD be more Moons with bads in it, "Moons of Twilight" and "Moons of Dawn"

I'll make a Moon Table when Big S approves of this idea